Chemical Calculations

Question 1

What is the equation to work out moles

Answer 1

Moles=mass/mr

Question 2

How do you work out reacting masses

Answer 2

1) work out molar ratio
2) Work out the number of moles for the substance you have been given mass of by dividing mass by mr
3) substitute this into the molar ratio to find out the number of moles for the other substance
4) times the number of moles found by mr to work out mass of other substance

Question 3

How do you calculate percentage yield

Answer 3

1) Work out the maximum theoretical mass of the product using reacting masses
2) divide the actual mass obtained by the maximum theoretical mass

Question 4

Why is it not always possible to acheive 100% yield a chemical reaction

Answer 4

• Some of the product may be lost when it is separated from the reaction mixture
• The reaction may be reversible and not go to completion
• Some of the reactants may react in different ways to what is expected so we do not get the product we expect

Question 5

What is equation to work out percentage yield

Answer 5

Percentage yield = mass of product actually made/maximum theoretical mass of product x100

Question 6

Is it possible to achieve greater than 100% yield

Answer 6

It is not possible to achieve greater than 100% yield as this would mean atoms have been created

Question 7

What is atom economy

Answer 7

Atom economy is a measure of the amount of starting materials that end up as useful products

Question 8

Why is atom economy important

Answer 8

• By minimising the production of unwanted products , we save money
• We increase sustainability by not wasting resources

Question 9

How do you calculate atom economy

Answer 9

Atom economy = rfm of desired products (from equation) / sum of rfm of all reactants (from equation

• rfm=relative formula mass
• from equation means that big numbers count

Question 10

Is an atom economy of over 100% possible ?

Answer 10

An atom economy of over 100% is not possible as that would mean we are creating atoms which is not possible

Question 11

What is meant by concentration

Answer 11

Concentration is the mass of a solute in the given volume of solution

-The unit for this definition is g/dm3

Question 12

What is the first equation for concentration

Answer 12

Concentration=mass(g)/volume(dm3)

Question 13

What happens if you increase the mass of a solute and keep the volume the same

Answer 13

If you increase the mass of a solute and keep the volume the same the concentration increases

Question 14

What happens if you increase the volume of solution but keep the mass of the solute the same

Answer 14

If you increase the volume of solution but keep the mass of solute the same you decrease concentration

Question 15

What is another meaing for concentration

Answer 15

• Concentration is the number of moles of a solute in a given volume of solution
• unit is mol/dm3

Question 16

What is the equation for working out this kind of concentration

Answer 16

Concentration(mol/dm3)=no of moles/volume (dm3)

Question 17

How do you work out titration calculations

Answer 17

1) work out the molar ratio
2) write c= v=n= under the solutions you are trying to work out
3) plug in what you know
4) use the equation(concentration=no of moles/volume)to work out the moles of one solution
5) use the molar ratio to work out the number of moles of the other solution
6) use the equation again to fill in any unknowns

Question 18

How do you work out the concentration g/dm3 from the concentration in mol/dm3

Answer 18

To work out the concentration in g/dm3 from mol/dm3 you multiply the conc in mol/dm3 by the rfm of the substance

Question 19

What volume does a mole of any gas occupy at room temperature and pressure

Answer 19

One mole of any gas occupies a volume of 24dm3 (at room temperature and pressure

Question 20

How do you work out the volume of a gas

Answer 20

Volume =no of moles x24

Question 21

How do you work out the volumes of gasses in a chemical reaction

Answer 21

1) You work out the molar ratio
2) work out the no of moles of the substance you have been given the mass for (using no of moles=mass/mr)
3) subsitute this into the molar ratio to find the amount of moles of the the other substance
4) use the no of moles x 24 to work out the volume of the other substance

Question 22

What is the limiting reactant

Answer 22

The limiting reactant is the reactant that is fully used up in the chemical reaction

Question 23

What effect does the limiting reactant have on the amount of products produced

Answer 23

The moles amount of product produced will be the same as the limiting reactant if the ratio is 1:1

Question 24

What is the limiting reactant

Answer 24

The limiting reactant is the reactant with the lower number of moles