3 quantitative chemistry

Question 1

what is the law of conservation of mass?

Answer 1

the law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.

Question 2

write a balanced equation of magnesium reacting with hydrochloric acid.

Answer 2

Mg(s) + 2HCl(aq) –> MgCl2(aq) + H2(g)

Question 3

define relative atomic mass and relative formula mass.

Answer 3

RAM - average mass of atoms in an element taking into account masses and abundance of its isotopes, relative to 12C.
RFM - sum of RAM’s of all atoms in the formula.

Question 3

use equation to explain why carbon dioxide and water escape from the test tube.

Answer 3

they are both gases.

Question 4

what is the relative formula mass of:
CaF2
C6H12O6

Answer 4

CaF2 - (Ca = 40, F = 19)
40 + 19 + 19 = 78
C6H12O6 - (C = 12, H = 1, O = 16)
(12 x 6) + (1 x 12) + (16 x 6) = 180

Question 5

what is Avogadro’s constant?

Answer 5

the number of atoms, molecules or ions in a mole of a given substance. the value of the constant is 6.02 x 10^23.

Question 5

calculate the amount of carbon dioxide in moles in 0.32g of carbon dioxide. relative atomic masses (Ar): carbon = 12, oxygen = 16

Answer 5

moles = mass/Mr
0.32/44 = 0.007

Question 6

what is the formula that links mass, molecular mass and moles together.

Answer 6

mass = Mr x moles

Question 7

what is the mass of 20 moles of calcium carbonate, CaCO3

Answer 7

mass = Mr x moles
Mr = 100
100 x 20 = 2000g

Question 8

hydrogen peroxide decomposes in water to form water and oxygen. how much oxygen will be given off from 40.8g of hydrogen peroxide?

Answer 8

step 1: write the balanced equation 2 H202(l) –> 2 H2O + O2(g) Mr of H2O2 = 34
step 2: number of moles in 40.8g: 40.8/34 = 1.2 moles
ratio in the balanced equation of H2O2 : O2 = 2 : 1
step 3: therefore number of moles of O2 = 0.6 moles
step 4: mass of oxygen = 0.6 x 32 (Mr of O2) = 19.2

Question 8

state what we mean by a limiting reactant in a chemical reaction.

Answer 8

in a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used. the reactant that is completely used up is called the limiting reactant because it limits the amount of products.

Question 9

what is the molar volume of a gas at room temperature and pressure?

Answer 9

1 mole of a gas at room temperature and pressure occupies 24 dm^3

Question 9

what is titration?

Answer 9

a technique for finding the concentration of a solution by reacting a known volume of this solution with a solution of known concentration.

Question 10

write down the two formulae that link concentration, mole/mass and volume together.

Answer 10

concentration (g per dm^3) = mass (g)/volume (dm^3)
concentration (mol per dm^3) = nr of moles/volume (dm^3)

Question 11

why is it not always possible to obtain the theoretical amount of product in a chemical reaction?

Answer 11

• the reaction may not go to completion because it is reversible.
• some of the product may be lost when it is separated from the reaction mixture.
• some of the reactants may react in ways different to the expected reaction (side reactions may occur).

Question 11

how do you conduct a titration?

Answer 11

a) rinse the pipette with a solution of unknown concentration. use the pipette to measure out the known volume of this solution.
b) add an indicator (a substance that changes colour at the end of titration)
c) rinse the burette with a solution of known concentration. discard the liquid. use a burette to gradually add the solution of a known concentration
d) when indicator changes colour (at the end point), the volume added is recorded
e) it is important to get concordant volume results - they have to lie close to each other
f) suitable calculations are performed to find the concentration

Question 12

what is the % yield of NH3 if 40.5g NH3 is produced from 20.0 mol H2 and excess N2?

Answer 12

step 1 - write a balanced equation
N2 + 3 H2 –> 2 NH3
step 2 - calculate the theoretical amount of NH3 moles NH3 (ratio of H2 to NH3 is 3 : 2); of 20/1.5 = 13.3 moles
13.3 x 17 (Mr of NH3) = 227
step 3 - calculate percentage yield of NH3
40.5/227 x 100 = 17.8%

Question 13

how is the percentage yield of a product in a chemical reaction?

Answer 13

% yield = actual mass of a product/maximum theoretical mass of product x 100%

Question 14

what is atom economy?

Answer 14

a measure of the amount of starting materials that end up as useful products. it is a ratio of the relative formula mass of desired product to the sum of relative formula masses of reactants.