Calculations

Question 1

what does it mean if a reactant is in ‘excess’

Answer 1

some left over/ extra after the reaction has occured

Question 2

why is it called a ‘limiting’ reagent

Answer 2

limits how much product can be formed

Question 3

what is the mass of 1 mole

Answer 3

gfm

Question 4

steps in excess calculations

Answer 4

1. find no of moles for each reactant
2. write down molar ratio
3. sub in actual no of moles of reactant 1 and see how much reactant 2 is needed
4. if have enough then r2 is in excess and r1 is limiting reagent, if not sub in other way around

Question 5

steps on how to find mass of product formed

Answer 5

1. work out which reactant is in excess and which is limiting reagent
2. work out no of moles of product will be produced based on moles of LIMITING REAGENT
3. use mass,n,gfm triangle to calculate mass of product formed

Question 6

purpose of percentage yield calculations

Answer 6

chemical reactions do not produce 100% of the product expected and the actual yield is less than this.

Question 7

percentage yield equation

Answer 7

%yield=actual yield/theoretical yield x100

Question 8

what is generally considered to be a good %yield?

Answer 8

above 70%

Question 9

steps for percentage yield calculation?

Answer 9

1. write out molar ratio
2. work out number of moles of reactant
3. work out number of moles of product
4. work out mass of product (theoretical yield)
5. find %yield using theoretical and actual yield

Question 10

what is molar volume?

Answer 10

the volume occupied by one mole of gas

Question 11

units of molar volume

Answer 11

litres/mol

Question 12

the molar volume is the same…

Answer 12

for all gases at the same temperature and pressure

Question 13

approximate molar volume of gases at 20 degrees is…

Answer 13

24 litres/mol

Question 14

approximate molar volume of gases at 0 degrees is…

Answer 14

22.4 litres/mol

Question 15

molar volume equation

Answer 15

molar volume = volume/no. of moles

Question 16

determining molar volume experimentally

Answer 16

1. work out mass of gas (mass of flask+gas-mass of flask)
2. work out no. of moles n=m/gfm
3. sub in no. of moles to molar volume equation

Question 17

way to work out mass of reactant mixture without calculator

Answer 17

molar ratio

Question 18

reacting volumes (gas in a reaction) steps

Answer 18

1. calculate no of moles of substance in the question
2. use molar ratio
3. use molar volume relationship

Question 19

excess and molar volume steps

Answer 19

1. find the limiting reagent
2. use molar ratio to find moles of what you are looking for
3. use molar volume relationship

Question 20

2 ways the efficiency of reactants being converted into products can be measured:

Answer 20

1. percentage yield

2. atom economy

Question 21

%yield: after a chemist makes a new product…

Answer 21

they allow the product to ‘dry’ so that any water/solvent is removed.
watch glass weighed again with new substance on - difference in masses = actual mass of product
can then carry out %yield calculation

Question 22

percentage yield provides a measure of…

Answer 22

the degree in which the limiting reagent is converted to the desired product

Question 23

what does percentage yield not take into account

Answer 23

by-products formed that could be problematic and wasteful

Question 24

atom economy measures the…

Answer 24

proportion of the total mass of reactants converted into the desired product.

Question 25

reactions which have a high percentage yield may have a low atom economy if…

Answer 25

large quantities of unwanted by-products are formed

Question 26

atom economy equation

Answer 26

%atom economy= mass of desired products / total mass of reactant x100

Question 27

100% atom economy means that…

Answer 27

all reactants are turned into products. This is desirable

Question 28

Situation where there is 100% atom economy

Answer 28

if there is only 1 product

Question 29

steps for calculating atom economy

Answer 29

1. work out mass of desired product (no. of moles x gfm)
2. work out total mass of reactants
3. atom economy equation

Question 30

clean by-products that are easy to dispose of

Answer 30

water

Question 31

comparing two processes

Answer 31

work out atom economy for each and compare by-products - are they wasteful, toxic, flammable etc?

Question 32

steps for redox titration calculations:

Answer 32

1. work out no. of moles of substance of known concentration
2. use molar ratio to work out no. of moles of substance of unknown concentration
3. calculate concentration using c=n/v