Unit 3: quantitative chemistry

Question 1

what is the relative formula mass (Mr) of a compound?

Answer 1

the sum of the relative atomic masses of all the atoms in the compound

Question 2

what is avogadro’s constant?

Answer 2

6.02 x 10^23

Question 3

how much will one mole of carbon weigh?

Answer 3

12 g

Question 4

how do you find the amount of moles in a given mass?

Answer 4

moles of substance = mass/Mr

Question 5

in a chemical reaction, is mass always lost, gained or conserved?

Answer 5

conserved

Question 6

why might there appear to be a change in mass in a chemical reaction?

Answer 6

there is a gas involved in the reaction

Question 7

what is a limiting reactant?

Answer 7

the reactant that will be used up first in a chemical reaction

Question 8

calculate the mass of aluminium oxide formed when 125g of aluminium is burned in air?

Answer 8

225g of aluminium oxide

Question 9

how much space will one mole of any gas occupy at 20C?

Answer 9

24dm^3

Question 10

how do you find the volume of a known mass of any gas at room temperature?

Answer 10

volume of gas= mass of gas/Mr of gas x 24

Question 11

what is concentration?

Answer 11

the amount of a substance in a certain volume of a solution

Question 12

how do you calculate concentration?

Answer 12

concentration = mass(or number of moles) of solute/volume of solvent(in dm^3)

Question 13

a student started with 30cm^3 of sulfuric acid of unknown concentration in a flask. She found that it took an average of 25 cm^3 of 0.1 mol/dm^3 sodium hydroxide to neutralise the sulfuric acid. Find the concentration of the acid in mol/dm^3. The equation is: 2NaOH + H2SO4 = Na2SO4 + 2H2O

Answer 13

0.417 mol/dm^3

Question 14

how do you convert from cm^3 to dm^3?

Answer 14

divide cm^3 by 1000

Question 15

What is the atom economy of a reaction?

Answer 15

it is the how much of the mass of the reactants is wasted when manufacturing a chemical and how much mass ends up as a useful product

Question 16

how do you calculate atom economy?

Answer 16

atom economy= Mr of desired products/Mr of all reactants x 100

Question 17

calculate the atom economy of the following reaction to produce hydrogen gas. CH4 + H2O = CO + 3H2

Answer 17

17.6%

Question 18

Why is high atom economy good?

Answer 18

it reduces waste. Reactions with low atom economy use up resources very quickly and create lots of waste that needs to be disposed of somehow, making these reactions unsustainable.
it is cheaper than reactions with low atom economy.

Question 19

What is percentage yield?

Answer 19

it is a percentage that tells you about the overall success of an experiment. it compares what you think you should get (theoretical yield) with that you get in practice (actual yield)

Question 20

how do you calculate percentage yield?

Answer 20

percentage yield = mass of product actually made (g)/maximum theoretical mass of product(g) x 100

Question 21

Why are percentage yields always less than 100%?

Answer 21

1) in reversible reactions, the products can turn back into the reactants, so the yield will never be 100%
2) the reactants might react differently to how you expect. They might react with gases in the air or impurities in the reaction mixture, so they end up forming extra products other than the ones you want
3) you lose some product when you separate it from the reaction mixture or transfer it from one container to another

Question 22

why do you want to have a high percentage yield?

Answer 22

it reduces waste and costs