Chemistry- Quantitative Chemistry

The molecular mass of methane, CH4, is 16. The atomic mass of carbon is 12. What is the percentage mass of carbon in methane?

(12*16) x 100= 0.75 x 100= 75%.

Magnesium reacts with hydrochloric acid as follows: Mg + 2HCl —

0.4 moles. The equation shows you get 1 mole of MgCl2 for each mole of Mg that reacts. The acid is in excess, so all 0.4 moles of magnesium will react to make 0.4 moles of MgCl2.

The formula of magnesium bromide is MgBr2. Atomic mass of Mg= 24, Br= 80. What mass of bromine is there in 0.25 moles of MgBr2?

1 mole of MgBr2 contains 2 moles of Br atoms, so mass of Br in 1 mole of MgBr2 = 2 x 80 =160g. So mass of Br in 0.25 moles = 160 * 4 = 40g.

What mass of sodium sulfate is there in 100cm3 of sodium sulfate solution if the solution has a concentration of 5g/dm3?

100cm3 = 100 * 1000 = 0.1dm3 Mass = concentration x volume = 5 x 0.1 = 0.5g.

The atomic mass of oxygen is 16 and the atomic mass of nitrogen is 14. Find the molecular mass of nitric oxide, NO.

14+16=30. There's only one atom of each element in the formula, so just add the atomic mass values together.

The formula of the compound sulfur hexafluoride is SF6. Calculate the molecular mass of the compound. F=19 S=32.

32 + (19x6)= 32 + 114= 146

The reaction between sodium and water: 2Na + 2H2O —

4 : 3. Every 2 + 2 = 4 moles of products makes 2 + 1 = 3 moles of products.

Find the molecular mass of calcium hydroxide, Ca(OH)2. Ca=40, O=16, H=1.

40 + [2 x (16 + 1)] = 40 + (2 x 17)= 40 + 34= 74.

Compound Z decomposes, it produces 46g of sodium and 84g of nitrogen. Deduce the balanced equation for decomposition of Z, in terms of Z, N2 and Na. Atomic mass of Na= 23, N= 14.

46g of Na = 46 * 23 = 2 moles. 84g of N2 = 84 * (2 x 14) = 84 * 28 = 3 moles. The numbers of moles are all whole numbers, so the balanced equation is: 2Z —

A large bottle of a popular brand of soft drink contains 130g of sugar. The bottle holds 2 dm3 of the drink. What is the sugar concentration of the drink in g/dm3?

Concentration = mass * volume = 130 * 2 = 65g/dm3.

0.9 moles compound A react completely with 0.6 moles of compound B to form 0.6 moles of compound C, which is the only product of this reaction. Find the balanced equation for this reaction in terms of A,B and C.

Divide all the number of moles by the smallest number of moles: A: 0.9 * 0.6 = 1.5. B: 0.6 *0.6 = 1. C: 0.6 * 0.6 = 1. Now multiply to get them all to be whole numbers. So in this case, multiply by 2: A: 1.5 x 2 = 3. B: 1 x 2 = 2. C: 1 x 2 = 2. So the balanced equation is : 3A + 2B —

How do you find the concentration of a solution in g/dm3?

Divide the mass of solute in grams by the volume of the solution in dm3.

Find the concentration, in g/dm3, of the solution formed when 15g of potassium nitrate are dissolved in 250cm3 of water.

First convert the volume to dm3. 250 * 1000 = 0.25dm3/ So concentration = mass * volume = 15 * 0.25 = 60g/dm3.

What is the definition of a mole?

One mole is 6.02 x 10(to the power of 23) particles of a substance. (Avogadro constant).

Zinc reacts with oxygen to form zinc oxide, ZnO like this: 2Zn + O2 —

The equation makes 2 moles of ZnO, so to make 8 moles of ZnO, multiply by 8 * 2 = 4 : 8Zn + 4O2 —> 8ZnO, so making 8 moles of ZnO uses 8 + 4 = 12 moles of reactants.

The equation for reaction between sulfuric acid and sodium carbonate is : H2SO4 + Na2CO3 —

The mass will decrease. Both reactants are in solution, so both their masses will be included in the mass of the beaker at the start of experiment. However, one of the products is carbon dioxide gas, which will escape the open beaker.

A student dissolved some sodium chloride in water to form a solution. She then made a second solution, using the same mass of sodium chloride, but only half as much water. How will the concentration of the second solution compare with the first?

Halving the volume of the solution while keeping the mass of sodium chloride the same will double the concentration.

How does the balanced symbol equation for a reaction to show that mass is conserved in that reaction?

In a balanced symbol equation, the sum of the relative masses of the reactants will equal the sum of the relative masses of the products. There will be the same number of each type of atom on both sides of the equation.

Compound X has a molecular mass of 30 and contains 10% hydrogen by mass. Atomic mass of hydrogen is 1. How many hydrogen atoms are there in the molecular formula of compound X?

Mass of H = 10% of the molecular mass of X = 10% of 30 = 3. So the number of H atoms = 3 * 1 = 3.

What's the mass of 4 moles of potassium fluoride, KF? Atomic mass of K= 39, Atomic mass of F= 19.

Mass= number of moles x molecular mass 4 x (39 + 19) = 4 x 58 = 232g.

Fluorine reacts with water to form hydrogen fluoride, HF. The reaction equation is 2F2 + 2H2O —

Molecular mass of F= 2 x 19 = 38, so moles of F2 reacting = mass * molecular mass = 76 * 38 = 2. From the equation, 2 moles of F2 produces 4 moles of HF. Molecular mass of HF = 1 + 19 = 20, so mass of HF produced = moles x molecular mass = 4 x 20 = 80g.

3 moles of a compound A have a mass of 126g. What is the molecular mass of compound A?

Molecular mass= mass * number of moles = 126 * 3 = 42.

How do you calculate the percentage mass of an element in a compound?

Multiple the atomic mass of the element by the number of atoms of that element in the formula of the compound. Then divide by the compound's molecular mass and multiply by 100.

Which contains more particles, a mole of water or a mole of oxygen gas?

Neither- they both contain the same number of particles. 1 mole of water contains more atoms, but there are 6.02 x 10 (to the power of 23) molecules of H2O or O2 in 1 mole.

Methane, CH4, burns according to the following equation: CH4 + 2O2 —

Number of moles in 64g of CH4 = mass * molecular mass = 64 * 16 = 4. From the equation, each mole of CH4 makes 2 moles of H2O, so 2 x 4 = 8 moles of H2O are made. So mass of H2O produced = moles x molecular mass = 8 x 18 = 144g.

The molecular mass of sodium hydroxide, NaOH, is 40. How many moles are there in 500g of NaOH?

Number of moles= mass * molecular mass 500 * 40 = 12.5 moles.

What's the formula for the number of moles in a given mass?

Number of moles= mass in grams * molecular mass.

A scientists places a lump of a metal in an open container on a mass balance, then forgets to put it away before locking up the lab and going on holiday for two weeks, when the scientist returns, the reading on the mass balance has gone up. Suggest an explanation for this.

The metal must have reacted with one or more of the gases in the air, to form a solid or liquid product. The mass of the gas that reacted was not recorded by the balance before the reaction because the system wasn't closed.

What is the definition of limiting reactant?

The reactant that gets completely used up in a given reaction.

What is meant by term conservation of mass?

The total mass of the products in a reaction will always be equal to the total mass of the reactants. This is because no atoms are ever destroyed or created during a reaction.

How is the atomic mass or molecular mass of a substance related to the idea of moles?

The value of the atomic mass of an element or the molecular mass of a compound is equal to the mass in grams of 1 mole of the substance. For example, the molecular mass of CO2 is 44 so 1 mole of CO2 has a mass of 44g.

What does it mean if a reactant is said to be in excess?

There is more than enough of it present to allow the limiting reactant to be fully used up.

True or false? The molecular mass of a compound is always greater than the atomic mass of any of the elements in that compound.

True- you find the molecular mass by adding up the atomic mass of each element, so it must be greater.