C4 - calculations

Question 1

what is a mole

Answer 1

mass of a substance that contains 6.02x10^23

Question 2

give the value for Avogadro’s constant

Answer 2

6.02x10^23

Question 3

which formula is used to calculate the number of moles from mass and Mr

Answer 3

mass = moles x Mr

Question 4

what is a limiting reactant

Answer 4

the reactant that is completely used up in a chemical reaction

Question 5

what is a unit for concentration

Answer 5

g/dm3 or mol/dm3

Question 6

which formula is used to calculate concentration from mass and volume

Answer 6

volume(g/dm3) = mass(g)/ volume(dm3)

Question 7

how can you convert a volume reading in cm3 to dm3

Answer 7

divide by 1000

Question 8

if the amount of solute in a solution is increased, what happens to it’s concentration

Answer 8

increases

Question 9

if the volume of water in a solution is increased what happens to the concentration

Answer 9

decreases

Question 10

what is the yield of a reaction

Answer 10

mass of product obtained from the reaction

Question 11

what is the theoretical yield of a reaction

Answer 11

maximum mass of the product that could have been produced

Question 12

why is the actual yield always less than the theoretical yield

Answer 12

• reaction may be reversible
• some of the product can be lost on separation
• unexpected side reactions between reactants

Question 13

what is the percentage yield

Answer 13

actual yield as a proportion of theoretical yield

Question 14

how is percentage yield calculated

Answer 14

actual yield
—————— x 100
theoretical yield

Question 15

what is atom economy

Answer 15

measure of how many atoms of the reactants end up as useful products

Question 16

why is a high atom economy desirable

Answer 16

results in less waste/is more sustainalble

Question 17

how is percentage atom economy calculated

Answer 17

Mr of useful product
—————————— x 100
Mr of all reactants

Question 18

how can the concentration in mol/dm3 be calculated

Answer 18

moles of solute/
volume

Question 19

what is titration

Answer 19

method used to calculate the concentration of an unknown solution

Question 20

what is the end-point

Answer 20

the point at which the reaction is complete (when the indicator changes colour) and no substance is in excess

Question 21

how should solution be added from the burette close to the end point

Answer 21

drop by drop, swirling in between

Question 22

why is a white tile used in titration

Answer 22

to see the colour change better

Question 23

what is a titre

Answer 23

volume of solution added from the burette

Question 24

what does one mole of any gas occupy at room tempereature and pressure

Answer 24

24dm3 or 24000cm2

Question 25

how to work out the Mr

Answer 25

the sum of the relative atomic masses of the atoms in the numbers shown in the formula. Relative formula mass (Mr) of CaCO3 = 40+12+(3x16)=100.

Question 26

how to work out the Ar

Answer 26

relative atomic mass = (abundance of isotope 1 × mass of isotope 1 ) +
(abundance of isotope 2 × mass of isotope 2)
/100