CH2. Amount of Substance
What is percentage atom economy?
(Mr of desired product/Sum of Mr's of all reactants) x 100% Tip: Any reaction where there's only one product will have a 100% atom economy.
What are the state symbols?
(s) solid (l) liquid (g) gas (aq) aqueous (anything dissolved in water, eg acids)
Advantages of high atom economies
- Raw materials are used more efficiently. This is more sustainable. - Less by-products so less time and money spent separating these from the desirable product. - Produce less waste and so benefit the environment as waste chemicals are often harmful to the environment - It can also be good for society if chemical companies can find easier, cheaper ways to mass produce medicines and other useful chemicals, as this may mean the products can be sold for lower prices and be made available to more people.
Titrations method
-used to find concentration -put acid or alkali in burette with known concentration -have acid or alkali with unknown concentration but known volume -use pipette to measure acid or alkali and put in conical flask -add few (2-3) drops of indicator in conical flask -add drop by drop until indicator colour changes (the endpoint) -read from burette the amount of acid / alkali used. read at eye level from bottom of meniscus. record to two decimal places -repeat until concordant results (0.1cm^3 away from each other)
0.475 g of CH3Br reacts with excess NaOH in the following reaction: CH3Br + NaOH --> CH3OH + NaBr 0.153 g of CH3OH is produced. What is the percentage yield?
1. Work out how many moles of CH3Br you have: Mr of CH3Br = 12.0 + (3 × 1.0) + 79.9 = 94.9 Number of moles CH3Br = mass ÷ Mr = 0.475 ÷ 94.9 = 0.00501... moles. 2. Work out how many moles of product you would expect to make: From the equation, you know that 1 mole of CH3Br produces 1 mole of CH3OH so 0.00501... moles of CH3Br will produce 0.00501... moles of CH3OH. Now calculate the mass of that many moles of product: Mr of CH3OH = 12.0 + (4 × 1.0) + 16.0 = 32.0 Theoretical yield = number of moles × Mr = 0.00501... × 32.0 = 0.160... g. 4. Now put these numbers into the percentage yield formula: percentage yield = actual yield/theoretical yield x 100 = (0.153... ÷ 0.160...) × 100 = 95.5%
How to write Ionic Equations?
1. write down the word equation for the reaction 2. write down the balance symbol equation with state symbols 3. ONLY SPLIT THE AQUEOUS COMPOUNDS INTO into their indiviual ions 4. Highlight the species which are UNCHANGED in going from reactants to products. These species are not involved in the reaction. They are called SPECTATOR IONS. A change can be a change in state. 5. The remaining species (the ones that have changed) are the ones involved in the ionic equation. Rewrite the equation showing ONLY the species involved in the reaction. P.S if the element has a number in front to balance it, add that number infront of the split ions that make up that compound
What are standard conditions?
100kPa and 298K
How much CaO can be made when 34g of Ca is burnt completely in oxygen?
2Ca(s) + O2(g) -> 2CaO(s) !. Write out equation and balance it. 40x2 = 80 56x2 = 112 2. Work out the Mr/Ar of species involved. Write these as masses in g. 80g = 112g 1g =. 1.4g 34g = 47.6g 3. Divide the Ca side by 80 to find 1g then multiply by 34 to get 34g. Do the same for the CaO side. 47.6g is the theoretical mass of CaO we can produce.
What volume of H2 is produced when 12g of Potassium reacts with Water at 100kPa of pressure and 298K. Gas constant is 8.31JK^-1mol^-1.
2K(s) + 2H2O(l) -> 2KOH(aq) + H2(g) 1. Write out equation and balance it. Moles = Mass(g) / Ar Moles Moles = 12g/39 = 0.31 moles of K 2. Work out number of moles of Potassium. 2 moles of K react to produce 1 mole of H2 Moles of H2 is 0.31/2 = 0.155 moles of H2 3. Use equation to find out the molar ratio of K:H2 pV=nRT Re-arrange to get V=nRT/p V= (0.155 x 8.31 x 298)/100,000 So V= 3.84x10^-3m^3 4. Use the ideal gas equation to work out the volume.
What is Avogadro's number?
6.02 x 10^23 1 mole
Define Mole
A way of measuring the amount of a substance. Shortened to 'mol'
How to calculate volume of gases from an equation?
Balanced equations can be used to work out the volume of gases
A hydrocarbon combusts completely to make 0.845f of CO2 and 0.173g of H2O. What is the empirical formula of the hydrocarbon?
CO2. H2O. 1. Write down H2O and CO2 as headings (*The Carbon is CO2 and the Hydrogen in H2O can only come from the Hydrocarbon*) 0.845g. 0.173g. 2. Write down the masses of each molecule 0.845g/44.0 = 0.019. 0.173g/18.0 = 0.0096 3. Divide these by Relative Molecular Mass to get number of moles (*1 mole of CO2 has 1 mole of C atoms. Original hydrocarbon must have 0.019 moles of C atoms*) (*1 mole of H2O has 2 moles of H atoms. Original hydrocarbon must have 0.0096 x 2 = 0.0192 moles of H atoms.) 0.019/0.019 = 1 0.0192/0.019 = 1 4. Divide the number of C and H atoms by the smallest number of moles. Empirical Formula = CH
How to calculate the number of moles in a solution?
Concentration (moldm^-3) x Volume (dm^3)
Bromomethane is reacted with sodium hydroxide to make methanol: CH3Br + NaOH -> CH3OH + NaBr Calculate the percentage atom economy for this reaction.
First, calculate the total mass of the reactants — add up the relative molecular masses of everything on the left side of the balanced equation: Total mass = (12.0 + (3 × 1.0) + 79.9) + (23.0 + 16.0 + 1.0) = 134.9 Then find the mass of the desired product — that's the methanol: Mass of desired product = 12.0 + (3 × 1.0) + 16.0 + 1.0 = 32.0 Now you can find the % atom economy: % atom economy = (molecular mass of desired product/sum of molecular masses of all reactants) x 100 = 32.0 × 100 = 23.7%
15.7cm^3 of 0.450 moldm^-3 H2SO4 was required to neutralise 0.120 moldm^-3 of NaOH. Calculate the volume of NaOH being neutralised in cm^3.
H2SO4 + 2NaOH -> Na2SO4 + H2O 1. Write out equation and balance it. Moles = Conc (moldm^-3) x Vol (dm^3) Moles = 0.45 x (15.7x10^-3) = 7.07 x 10^-3 mol 2. Calculate the number of moles of H2SO4 1:2 ratio between H2SO4 and NaOH Moles of NaOH = 2 x Moles of H2SO4 NaOH = 2 x (7.07x10^-3) = 0.0141 mol 3. Use the equation to find out the molar ratio in order to work out moles of NaOh Volume (dm^3) = Moles/Conc (moldm^-3) Volume= 0.0141/0.120 = 0.118 dm^3 Volume in cm^3 = 0.118 x 100 = 118cm^3 4. Now calculate volume using Vol= Moles/conc
18.3cm^3 of 0.25moldm^-3 HCl was required to neutralise 25cm^3 of KOH. Calculate the concentration of KOH.
HCl + KOH -> KCl + H2O 1. Write out equation and balance it. Moles = Concentration (molddm^-3) x Volume (dm^3) Moles = 0.25 x (18.3x10^-3) = 4.58 x 10^-3 mol (*Volumes MUST be converted to dm^3 first*) 2. Calculate the number of moles of HCl. 1:1 ratio between HCl and KOH Moles of HCl = Moles of KOH KOH = (4.58x10^-3)/(25x10^-3) = 0.18 moldm^-3 4. Now calculate concentration using Conc = Moles/Vol
What is atom economy?
How efficient a reaction is
How to calculate the number of moles in a specific volume of a gas?
Ideal gas equation - rearrange for moles
How to calculate the number of moles in a solid?
Mass (g) / Mr or Ar
A compound contains 23.3% Magnesium, 30.7% Sulfur and 46.0% Oxygen. What is the empirical formula of this compound?
Mg. S. O. 1. Write out elements involved. 23.3g. 30.7g. 46.0g. 2. Write the percentages as masses. 23.3g/24.3 = 0.96 30.7g/32.1 = 0.96 46.0g/16.0 = 2.88 3. Divide these by the Relative Atomic Mass to get the number of moles 0.96/0.96 = 1 0.96/0.96 = 1 2.88/0.96 = 3 1 : 1 : 3 4. Divide all the numbers by the smallest number of moles Empirical Formula: MgSO3
Calculate the number of moles of 200 cm^3 of 0.35 moldm^-3 HCl.
Number of moles = Concentration x Volume Number of moles = 0.35 x (200x10^-3) Number of moles = 0.07 mol
Calculate the number of moles of 23g of Gold. Give your answers to 2 significant figures.
Number of moles = Mass / Ar Number of moles = 23 / 197 Number of moles = 0.12mol (to 2 s.f)
How many particles make up 0.67 moles of Ammonia (NH3)?
Number of particles = Avogadro's Number x Number of Moles Number of particles = 6.02 x 10^23 x 0.67 Number of particles = 4.03 x 10^23
How to calculate the number of particles in a substance?
Number of particles = Avogadro's Number x Number of moles
Types of indicators that can be used in titrations
Phenolphthalein Acid - Colourless Alkali - Pink Methyl Orange Acid - Red Alkali - Yellow
What are Ionic Equations?
Show the ions that are formed in solution and show which particles are reacting.
What can titrations be used to work out?
The concentration of an acid or alkali.
Define limiting reactant
The first reactant that is used up in a reaction. When the limiting reactant is all used up, no more product can form, and the reaction stops.
What is an empirical formula?
The simplest whole number ratio of atoms of each element in a compound
What is theoretical yield?
The theoretical yield is the mass of the product that should be formed in a chemical reaction. It assumes no chemicals are 'lost' in the process.
What are titrations used for?
Titrations are used to find the exact volumes of acid and alkali that react with each other
How to convert a Volume from cm^3 to dm^3?
To convert to dm^3 just divide by 1000 or add x10^-3 to the end of the number.
How to work out the molecular formula using the empirical formula?
To work out the molecular formula just work out Mr of empirical formula, divide by Mr of molecular formula. Use this number to multiply all the atoms in the empirical formula. STEP 1: Calculate the molar mass of the empirical formula. STEP 2: Divide the given molecular molar mass by the molar mass calculated for the empirical formula. STEP 3: Multiply all atoms by the whole number that resulted from step 2. This is now the molecular formula.
How to calculate a theoretical mass/yield from an equation?
Using balanced equations to calculate theoretical mass
Define excess reactant
What is left over after the reaction stops because the limiting reactant got all used up.
How to calculate percentage yield?
actual yield/theoretical yield x 100
What should be on the left and right of a final ionic equation?
balanced charges
Calculate the molecular formula of a compound with the empirical formula C2H2O and a molar mass of 126.12 g/mol.
molar mass of molecular formula/molar mass of empirical formula: 126.12/(2 x 12.01) + (2 x 1.01) + 16.0 = 42.04 = 126.12/42.04 = 3 C2H2O --x3--> C6H6O3 Molecular Formula: C6H6O3
Calculate the volume in cm^3 of 0.36 moles of a gas at 100kPa and 298 K.
pV = nRT Rearrange to give V = nRT/p V = 0.36 x 8.31 x 298 / 100,000 V = 8.91 x 10^-3 m^3 V= 8.91 x 10^-3 x 1,000,000 = 8910cm^3
What is the Ideal Gas Equation?
pV=nRT p = pressure in pascals, Pa V = Volume in m^3 n = number of moles, mol R = 8.31 the gas constant (given in exam) T = temperature measured in kelvin, K
How to calculate theoretical yield?
theoretical yield = number of moles x Mr