Topic 5) Formulae, equations and moles

how to calculate the concentration of very dilute solutions? how to convert ppm into percentage and vise versa? 4mg of NaCl is dissolved in 8kg of solutions. What is the concentration of NaCl in ppm?

'parts per million' mass of solute per million grams of solvent. ppm = mass of solute / mass of solution x 10^64 mg = 0.004g -> m of solute 8kg -> g = 8000g -> m of solution 0.004/8000 x 10^6 = 0.5 ppm Converting ppm into a percentage: - /10^6 = ____ x 100 Converting a percentage into ppm: - /100 = _____ x 10^6

Be able to relate ionic and full equations, with state symbols, to observations from simple test tube reaction: i. Displacement reactions eg. Zinc reacts with copper sulfate solution. eg. Chlorine reacts with potassium iodide

- Metals react with ions of metal below them reactivity series. - Halogens react with a ions of a less reactive halogen. eg. Fe + CuSO4 - blue solution -> colourless + red solid produced Full equation: Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s) Ionic equation: Zn(s) + Cu2+(aq) -> Zn2+ (aq) + Cu(s) - eg. Cl + KI - give a red-brown solution Full equation: Cl2(g) + 2KI(aq) -> I2(aq) + 2KCl(aq) Full ionic equation: Cl2 (g) + 2I- -> i2(aq) + 2Cl-

what is the overview of this practical? (CPAC 3)

- find unknown conc of HCl (dilution technique)- NaOH + HCl -> NaCl + H2O

what are specific errors seen in titrations

- not rinsing all the solid from weighing boat - not rinsing stirrer - not shaking volumentric flask - not rinsing out burette & pipette with correct solution - not ensuring not air below tap of burette - getting air bubbles in stem of pipette - running solution too quickly and overshooting - not swirling flash after each addition

what is the overview of this practical? (CPAC 2)

- prep of standard solution from known mass of solid acid to find unknown conc of NaOH - end point determined by a colour change - concentration = moles/volumeEQ: NaOH + NH2SO3H -> NaNH2SO3 + H2O

what is the overview of this practical? (CPAC 1)

-> known masses of reactants combined -> gas -> MV = v of 1 mol of a gas at certain temp/pressure -> solids: n = m/mr gases: n=v/molar Full: CaCO3(s) + 2CH3COOH(aq) -> (CH3COO-)2Ca2+(aq) + CO2(g) + H20(l) I: CaCO3(s) + 2H+(aq) -> Ca2+(aq) + H2O(l) + CO2(g)

how to calculate the concentration of unknown solution using CPAC 2 results?

1) conc sulfamic acid (mass/mr) 2.5/97.1 = 0.0257mol -> conc = n/v = 0.0257/(250/1000) = 0.103 moldm3 2) mean titre - (23.40+23.50)/2 = 23.45cm3 3) n acid in titre 0.103 x (23.45/1000) = 0.0024mol 4) ratio to find conc of NaOH - 1:1 ratio - 0.0024 of sodium hydroxide - conc = 0.0024/(25/1000) = 0.096 moldm3 NaOH

Solution containing 12.8g of sulfuric acid (Mr=98.1) and 10.0g of sodium hydroxide (Mr = 40) are mixed and produce sodium sulfate and water according to the following equation: 2NaOH + H2SO4 -> Na2SO4 + 2H2O Calculate the mass of sodium sulfate (Mr: 142.1) produced

1) nNaOH = 10.0/40 = 0.250, Na2SO4:NaOH = 1:2 nNa2SO4 = 0.250/2 = 0.125mol 2) nH2SO4 = 12.8/98.1 = 0.1305mol, Na2SO4:H2SO4 (1:1) = 0.130mol 3) The reagent that produces the least product (0.125) is NaOH = limiting reagent 4) Mass of Na2SO4 = 0.125 x 142.1 = 17.7625g

how to calculate measurement uncertainties and measurement errors in experimental results A student using scales measure to nearest 0.05g. He zeros the scale measures 1.35g of solid. Calculate uncertainty

1) non digital equipment = half the smallest increment Eg. 18-19 (going up in 1, 1/2=0.5) 2) Digital = smallest digit (eg. 0.025, smallest = 0.001) adding/subtracting -> absolute uncertainties added dividing/multiplying -> percentage uncertainties added 2 readings = 0.00g and 1.35g Uncertainty of each reading = 0.05/2 = 0.025, total = 0.025x2 = 0.05g

how to write an ionic equation

1) take full equation and balance and write states 2) split (aq) state solutions into ions 3) cancel out spectator ions (not changing state or oxidation number)

what is the procedure of this experiment? (CPAC 1)

1. 30cm3 1moldm3 ethanoic acid in boiling tube 2. apparatus (measuring cylinder or gas syringe) 3. 0.05g CaCO3 into test tube, weigh test tube 4. remove bung, add contents, replace bung on BT5. during reaction, gas collected over water 6. when complete, measure volume of gas in MC 7. reweigh test tube with calcium carbonate 8. repeat exp increasing mass CaCO3 (max 0.40g) 9. plot graph mass of CaCO3 over volume CO2 10. line of best fit at origin (no CaCO3, no CO2)

how might this experiment be improved? (CPAC 2+3)

1. burette uncertainty of ± 0.05 = less accurate-> titre larger, increase volume + lower conc 2. weight scale = error of 0.05g = less accurate-> larger mass 3. Determining end point (subjective)

how to calculate the empirical formula from percentage composition

1. divide each percentage by RAM 2. divide results by smallest number 3. if not integers multiply both numbers to find integers

how to calculate the molecular formula from the empirical formula

1. empirical mass is worked out, if the same as molar mass than molecular + empirical are the same 2. if empirical and molar mass not the same then: -> molar mass / empirical mass

how might this experiment be improved? (CPAC 1)

1. high solubility of CO2 in water (larger masses of CO3 used, less CO2 dissolved). Instead: gas syringe or reaction forms hydrogen, less dissolving eg. Mg(s) + H2SO4(aq) -> MgSO4(aq) + H2(g) 2. gas loss before replacing bung. instead: use tube containing acid inside vessel containing CaCO3 3. Lack of precision in cylinder - systematic (±0.5) -> use smaller increments = more accurate

what are the two ways in calculating the empirical formula

1. mass percentage composition 2. masses from combustion

what are the different significant figures

1. non zero numbers are always significant (1.23 = 3sfg) 2. inbetween zeros are always significant (1.0005 = 5 sgf) 3. leading zeros are never significant (0.000055 = 2 sgf) 4. trailing zeros are sometimes significant (70,000 = 1 sgf but 70,000. = 5 sgf)

what is the procedure of this experiment? (CPAC 3)

1. transfer 25cm3 HCl using volumetric pipette to volumetric flask + make total 250 w distilled water 2. transfer 25cm3 solution into conical flask 3. add 2-3 drops of phenophthalein indicator 4. titrate contents of flask against standard NaOH 5. repeat until concordant

what is the procedure of this experiment? (CPAC 2)

1. weigh 2.5g sulfamic acid in test tube + weigh 2. acid to 100cm3 distilled h20 to 250cm3 beaker 3. reweigh empty test tube (mass dif = mass of acid) 4. all acid dissolved + put into 250 volumetric flask 5. make up final 250cm3 with distilled water 6. with pipette transfer 25cm3 of NaOH into flask 7. drops of phenophalien or methyl-orange 8. titrate conical flask against standard (0.05cm3) 9. get 2 concordant titres into suitable table

25cm3 of a 0.75mol dm^-3 solution of sodium nitrite was placed in a conical flask and weighed. Using a pipette 25cm3 of a 1.0mol dm3 solution of ammonium chloride was added and the flask quickly connected to a gas syringe and placed on an electric heater and gently warmed. The two solutions reacted giving off a gas. When all bubbles had ceased, the flask was weighed. The lab temperature and pressure were measured - Mass of flask + sodium nitrite = 143.65g - Mass of ammonium chloride solution = 25.00g - Mass of flask after reaction = 168.60g - Volume of has evolved = 45cm3 = 4.5x10^-6m3 - Room temperature = 18.5 = 291.5K - Room pressure = 101kPa = 101000Pa calculate molar mass, identity

143.65 + 25.00 = 168.65 -> before reaction = 168.60 -> after reaction = 0.05g difference -> mass lost during reaction Pv = nrt 101000 x 4.5x10^-6 = n x 8.31 x 291.5 N = 0.00188molMr = mass/moles = 0.05/0.00188 = 26.59574 = 27gmol^-1 The gas is likely to be nitrogen or an oxide of nitrogen. The one that most closely fits is nitrogen molar mass = 28gmol^-1 (N2)

calculate the number of molecules in 3.33g of methane, CH4

3.33/16 = 0.208moles number of particles = moles x avogadros constant number of particles = 0.208 x 6.02x10^23 number of particles = 1.25x10^23

what is avogadro's constant

6.02 x 10^23 equal to the number of carbon atoms in exactly 12g of the carbon-12 isotope -> 12g of carbon = 1 mole = 6.02x10^23 atoms of carbo eg. 1 mol of sodium contains 6.02x10^23 sodium atoms eg. 1 mol of water contains 6.02x10^23 water molecules eg. 1mol CaCl2 = 6.02x10^23 Ca2+ & 2x 6.02x10^23 Cl-

Be able to relate ionic and full equations, with state symbols, to observations from simple test tube reaction: ii. acid reactions eg. Zinc reacts with sulfuric acid eg. Calcium oxide reacts with HCl eg. sodium carbonate reacts with nitric acid

Acids react with metals -> bubbles of hydrogen + salt. React with bases to form a salt and water React with carbonates form salt + CO2 + H2O eg. Zn + H2SO4 Full: Zn(s) + H2SO4(aq) -> H2(g) + ZnSO4(aq) Ionic: Zn(s) + 2H+(aq) -> H2(g) + Zn2+(aq) eg. CaO + HCl Full: CaO(s) + 2HCl (aq) -> CaCl2(aq) + H2O(l) Ionic: CaO(s) + 2H(aq) -> Ca2+(aq) + H2O(l) : eg. Na2CO3 + HNO3 Full: Na2CO3(aq) + 2HNO3(aq) -> 2NaNO3 (aq) + CO2(g) + H20(l) Ionic: CO3^2- (aq) + 2H+ (aq) -> CO2 (g) + H20(l)

A compound contains 38.7% calcium, 20% phosphorus and 41.3% oxygen by mass. the molar mass of the compound is 310gmol. calculate the: - empirical formula - molecular formula

Ca: 38.7/40.1 = 0.97 P: 20/31 = 0.65 O: 41.3/16 = 2.58 Ca: 0.97/0.65 = 1.5 = 3 P: 0.65/0.65 = 1 = 2 O: 2.58/0.65 = 4 = 8 Empirical: Ca3P2O8 = 310.3 which is same as molar mass

how to find the molar volume of CO2 from CPAC 1? eg. with data from graph? (CPAC 1)

CaCO3(s) + 2H+(aq) -> Ca2+(aq) + H2O(l) + CO2(g) 1. using graph find VCO2 made from 0.25g CaCO3 = 0.25g CaCO3 = 34cm3 CO2 released 2. moles in CaCO3: mass/mr = 0.25/100.1 = 0.0025 3. moles released from CaCO3 (CO2): 0.0025 used, 1:1 ratio of CaCO3 + CO2 = nCO2 = 0.0025 4. Find MV of CO2: molar volume = volume / mole = 34/0.0025 = 13,600 = 13,600/1000 = 13.6 dm3 -> real value - 24dm3

difference between risks and hazards? what are some examples?

Hazard: can cause harm e.g. chemicals, noise, stress Risk: chance (high/low) that hazard will cause harm eg. exposure to a chemical substance prevent: googles, protective wear, materials, dilution of acid etc

Be able to relate ionic and full equations, with state symbols, to observations from simple test tube reaction: ii. precipitation reactions eg. barium sulfate made by solutions of barium chloride to dilute sulfuric acid (or any sulfate)

Insoluble salts can be prepared by adding solutions containing their constituent ions. eg. BaSO4 from sulfuric acid Full: BaCl2 (aq) + H2SO4(aq) -> BaSO4(s) + 2HCl(aq) Ionic: Ba2+(aq) + SO42-(aq) -> BaSO4(s)

using the moles equations for gases Calculate the volume of carbon dioxide produced when 2.68g of calcium carbonate is heated and decomposes according to the equation: CaCO3(s) -> CaO(s) + CO2(g)

Mol of calcium carbonate = 2.68/100.1 = 0.0268 mol 1:1 ratio with carbon dioxide = 0.0268mol Volume = moles x molar volume = 0.0268 x 24000 = 643.2cm3

how to calculate the concentration of a solution in gdm3

Moles = concentration x volume, gdm3 = x molar mass eg. C = n/v = 0.123/(250/1000) = 0.492moldm3 Molar mass NaOH = 40 = 0.492 x 40 = 19.68 gdm3

how to determine the water of crystallisation of a compound? eg. 0.100mol of hydrated sodium carbonate (Na2CO3.xH20) has a mass of 28.6g. Calculate its molar mass and hence the number of molecules in crystalisation

Mr = mass / moles = 28.6 / 0.100 = 286 Mass of Na2CO3 = 106.0 286 - 106.0 = 180 = 180g of water molecules in 1 mol of hydrated solid Moles = 180/18 = 10 Number of molecules = 10 = Na2Cl3.10H2O

what are the different types of errors in a experiment?

Random - incorrect values = different magnitudes = anomalous = reducing reliability. Repeats + mean = spot anomaly + remove (doesn't remove error) Systematic - incorrect values = always same magnitude (direction + size), do not affect trends but different to predictions = reduce accuracy so change equipment/ recalibrate/use it properly

Calculate the volume occupied by 0.781 mol of oxygen at a pressure of 220 kPa and a temperature of 21 °C

Step 1: Rearrange the ideal gas equation to find volume of gas -> V = nRT/p Step 2: Calculate the volume the oxygen gas occupies p = 220 kPa = 220 000 Pa n = 0.781 mol R = 8.31 J K-1 mol-1 T = 21 oC = 294 K V = 0.781 x 8.31 x 294 / 2200004 = 0.00867 m3 = 8.67 dm3

1) A sample of impure magnesium was analysed by allowing it to react with excess HCl solution. After 1.32 g of the impure metal was treated with 100 cm3 of 0.750 moldm-3 HCl, the remaining solution required 50 cm3 of a 0.250 moldm-3 aqueous sodium hydroxide for complete neutralisation. Assuming the impurities do not react with the acid, what is the percentage by mass of Mg in the sample? 2) A 1.00g sample of metal X (down to form 2+ ions) was added to 100cm3 of 0.500moldm3 h2so4. after all the metal had reacted the remaining acid required 33.40cm3 of 0.500 moldm3 NaOH solution for neutrlisation. calculate the molar mass of the metal and identify the element

[Ans: 57.5%] ans: Mg = 24

answer these questions about CPAC 1: a. what is the acid + base in reaction. why is base not an alkali? b. role of acid? c. what do you see as reaction proceeds? when is it finished? d. assumptions of why two gases occupy same v at same temp

a. ethanoic acid and calcium carbonate. alkali is soluble, base and CaCO3 = insoluble b. acid is in excess, reacting with CaCO3 -> CO2 c. slight effervescence, bubbling stop, colourless d. assumption that actual volume taken up by molecules is neglialble. distance bt moles in gaseous states - big = seen as points in space

answer these questions about CPAC 2: a. mass = ±0.01g volumetric flask = ±0.05. percentage uncertainty? b. calculate percentage error when using pipette c. calculate uncertainty of titre d. using answers from a+b find highest/lowest conc NaOH e. what happen to conc if you leave cap of NaOH?

a. mass ±0.01%, volumetric flask (±0.05/250) x100 = 0.02% sum = 0.01 + 0.02 = 0.03% b. pipette: (0.05/25) x 100 = 0.2% c. titre: (0.05/2)/23.45 x100 = 0.43%d. 0.03 + 0.2 + 0.43 = 0.46% d. max = 0.096 + (0.46/100) = 0.1006moldm3d. min = 0.096 - (0.46/100) = 0.0914moldm3 e. 1. evaporation of water = increase conc 2. reaction of NaOH w CO2 reduce conc

percentage error = Eg. 250cm3 flask has error ±0.25cm3 calculate percentage uncertainty

absolute uncertaintiy / readingormeasurment = 0.25/250 x100 = 0.1%

what is a mole

amount of substance containing Avogadro's constant (6.02x10^23) number of atoms/molecules/groups of ions

using the moles equations for solutions In a titration, neutralisation of 25.0cm3 of 0.100moldm3 hydrochloric acid solution required 26.8cm3 of sodium hydroxide solution. NaOH + HCl -> NaCl + H20 Calculate the concentration of sodium hydroxide Calculate the volume of 0.100moldm3 hydrochloric acid solution required to neutralise 25.0cm3 of 0.0567moldm3 sodium carbonate solution. The equation: 2HCl + Na2CO3 -> 2NaCl + H2O + CO2

c = nv so n = c/v nHCl = 0.100/(25/1000) = 0.0025mol 1:1 so nNaOH = 0.0025mol c = 0.00250 x (26.8/1000) = 0.0933moldm3 nNa2CO3 = 0.0567 x (25/1000) = 0.001418 ratio of HCl:Na2CO2 is 2:1 0.001418 x 2 = 0.002836mol of HCl volume = moles/concentration volume = 0.002836/0.100 = 0.0284dm3 x1000 = 28.4cm3

why are some ionic compounds soluble in water? what happens when they dissolve? Eg. Sodium Chloride in water Eg. Iron(III) sulfate

forces of attraction between - hydration - positive cation + slightly negative oxygen atom - negative anion + slightly positive hydrogen atom NaCl(s) + aq -> Na +(aq) + Cl-(aq) Fe2(SO4)3(s) + aq -> 2Fe3+(aq) + 3SO4^2-(aq)

what are the three test tube reactions?

i. Displacement reactions ii. Reactions of acids iii. Precipitation reactions

how to calculate the molar mass of volatile liquids and gases

ideal gas equation: Pv=nrt (If gas produced from solution, mass of the solution lost = mass of gas produced) P = pressure (Pa = Nm-2) -> 1kPa = 1000Pa V = volume (m3) - 1cm3 = 1 x 10-6m3, 1dm3 = 1 x 10-3m3 N = moles R = gas constant (8.31 JK-1mol-1) T = temperature (degrees + 273 = Kelvin) Molar mass from moles = mass/mr

what is the molar mass of a substance?

mass in grams of one mole of a substance in gmol-1 -> found by adding relative atomic masses together eg. sodium RAM = 23 so molar mass = 23gmol-1 eg. NaCl mass = 23+35.5 so molar mass = 58.5gmol-1

how to calculate the empirical formula from masses on combustion

mass of CO2 and H2O produced when known mass of substance is burnt. mass carbon = CO2 x 12/44. (or long way) mass hydrogen = H2O x 1/18. any oxygen found by subtraction. then worked out like normal empirical.

what is atom economy

measure of the proportion of reactant atoms that become part of the desired product (rather than by-products) in the balanced chemical equation.

using the moles equations for solids calculate the mass of sodium sulfate produced when 3.45g of sodium hydroxide is neutralised by dilute sulfuric acid: 2NaOH + H2SO4 -> Na2SO4 + 2H20

molar mass NaOH = 40 moles = mass/mr nNaOH = 3.45/40 = 0.08625 ratio of Na2SO4:NaOH = 1:2 nNa2SO4 = 1/2 x 0.08625 = 0.04313 molar mass Na2SO4 = 142.1 massNa2SO4 = n x mr = 0.04313 x 142.1 = 6.13g

how many electrons in 12g of carbon-12

moles of carbon-12 = 12/12 = 1 mole of carbon atoms every carbon contains 6 electrons total electrons = 6 x 6.02x10^23 = 3.6x10^24 e-

calculate the number of sodium ions in 2.22g of sodium sulfate Na2SO4 (molar mass = 142.1gmol^-1)

moles of sodium sulfate = 2.22/142.1 = 0.0156moles number of particles = moles x avogadros constant number of ion groups = 0.0156 x 6.02x10^23 = 9.39x10^21 number of sodium ions = 9.39x10^21 x 2 = 1.88x10^22

2.22g of organic compound X containing carbon, hydrogen and oxygen only was burnt in excess air and 3.26g of carbon dioxide, 1.33g of water were obtained. Calculate the empirical formula of compound X.

nCO2 = 3.26/44 = 0.0741 moles of CO2 nH20 = 1.33/18 = 0.0739 moles of H20 massC = 0.0741 x 12 = 0.889g of carbon massH = 0.0739 x 2 = 0.1478g of hydrogen massO = 2.22 - 0.889 - 0.1478 = 1.18g oxygen could also find percentage and go from there or: nC = 0.0741/0.0738 = 1 nH = 0.1478/0.0738 = 2 nO = 1.18/16 = 0.0738/0.0738 = 1 Compound X = CH2O

A) 1.23g of a hydrocarbon was burnt in excess oxygen. The water produced was absorbed in silica gel and the carbon dioxide adsorbed by calcium oxide. The mass of water produced was 1.58g and the mass of carbon dioxide was 3.87g. Calculate its empirical formula. B) 0.284g of the same liquid hydrocarbon was injected into a gas syringe. This was then placed in an oven at 150 degrees where it fully vapourised. Its volume was 84.3cm3 and pressure was 106kPa. Calculate the molar mass of the hydrocarbon and using answer a) evaluate its molecular formula.

nH20: 1.58/18 = 0.0878 moles of water nCaCO3: 3.87/44 = 0.088 of calcium carbonate massH = 0.088 x 2 = 0.1756g of hydrogen massC = 0.088 x 12 = 1.056g of carbon C: 0.088/0.088 = 1 H: 0.1756/0.088 = 2 empirical = CH2 Pv=nrt Pressure = 106 x 1000 = 106000Pa, Volume = 84.3 x 10^-6 = 8.43x10^-5m3 N = ? R = 8.31 T = 150 + 273 = 423K Pv/rt = n (106000x8.43x10^-5)/(8.31 x 423) = 0.0025mol Molar mass = mass / moles = 0.284/0.0025 = 113.6g CH2 = 14 113.6/14 = 8.1143 = 8 = C8H16

a compound contained 34.3% sodium, 17.9% carbon and 47.8% oxygen by mass. calculate its empirical formula

nNa = 34.3/23 = 1.49 nC = 17.9/12 = 1.49 nO = 47.8/16 = 2.99 Na = 1.49/1.49 = 1 C = 1.49/1.49 = 1 O = 2.99/1.49 = 2 NaCO2

when 1000g of sulfur dioxide is reacted with excess oxygen, 1225g of sulfur trioxide is produced: 2SO2 + O2 -> 2SO3 Calculate the percentage yield

nSO2 = 1000/64.1 = 15.60mol ratio of SO2:SO3 = 1:1 so theoretical amount of sulfur trioxide produced = amount of SO2 reacted = 15.60 molar mass SO3 = 80.1 theoretical yield = moles x molar mass = 15.60 x 80.1 = 1250g actual/theoretical x100 = 1225/1250 x 100 = 98%

how to convert from moles to number of particles

number of particles = amount of substance (moles) x avogadros constant eg. number of water molecules in 1.44 moles of water = 1.44 x 6.02x10^23 = 8.67x10^23

how to get a percentage composition the formula of hydrated copper (II) sulfate is CuSO4.5H2O. Calculate the percentage of water in this salt.

percent = part/whole x100 Molar mass = 249.5gmol-1 Mass of water = 90 90/249.5 x100 = 36.1%

what are the different state symbols

solid (s), liquid (l), gas (g), and aqueous (aq)

what are hydrated ionic compounds

solid ionic compounds that contain water of crystalisation eg. Na2Co3.10H20

what are the moles equations for: solids liquids (solutions) gas

solids: moles = mass/mr liquids(solutions): moles = concentration x volume gas: moles = volume/molar volume

how to calculate concentration of unknown solution using CPAC 3 results?

standard NaOH = 0.096moldm3 1. titre = 24.35+24.35+24.35 = 73.05 /3 = 24.35cm3 2. nNaOH = 0.096 x (0.02435) =2.4336x10^-3 2. 1:1 ratio with HCl nHCl =2.4336x10^-3 3. concHCl = 2.4336x10^-3/0.025 = 0.097344 4. concHCl in original 250cm3 = 0.097344 x 10 = 0.97344moldm3

what is a limiting reagent

substance that determines the theoretical yield of product in reaction -> reactant completely used up

what is a molecular formula

the number of atoms of each element in one molecule of the substance

what is the percentage yield how to calculate the percentage yield

the ratio of the actual yield to the theoretical yield % yield = actual yield/theoretical yield x 100

what is 1 mole of a substance equal to?

the relative molecular mass measured in grams eg. 4g of Helium = 1 mole = 6.02x10^23 atoms of helium eg. 8g of helium = 1 moles = 1.2x10^24 atoms of helium

what is an empirical formula

the simplest whole number ratio of the atoms of each element in the substance

method of a back titration? why is it used? eg. finding concentration of impure calcium carbonate

used when substance = insoluble/cannot titrate direct 1) known excess HCl reacts with unknown CaCO3 2) amount of unreacted HCl (pipette) determined by titration with NaOH (burette) known concentration. 3) titration results -> amount of unreacted HCl + the amount CaCO3 can be found (stoichiometric calculations)

what is the data analysis of a back titration? eg. finding concentration of impure calcium carbonate

work backwards 1) write and balance equations for both reactions 2) calculate mol NaOH (mol=cv) 3) using ratio find mol HCl in excess (after reaction) 4) work out total mol acid (from question) 5) total - excess = reacted amount of acid 6) molar ratio to find mol of CaCO3

what are the different uncertainties of different equipment's

• balance -> 0.001 g • volumetric flask -> 0.1 cm3 • 25 cm3 pipette -> 0.1 cm3 • burette -> 0.05cm3 (uncertainty for each reading of +/- 0.05 cm3 then during a titration 2 readings, uncertainty on titre volume = +/- 0.10 cm3)

calculate the atom economy for the reaction producing hydrogen CH4(g) + H2O(g) → 3H2(g) + CO(g)

∑ Mr (useful products) / ∑ Mr (total of all reactants) x100 6/34 x 100 = 17.6%

how to calculate the atom economy

∑ Mr (useful products) / ∑ Mr (total reactants) x100