Calculation of excess

In an excess calculation you will be given

the quantities of two reactants

We can tackle excess calculations in four steps

Copy the equation and under the reactants write thenumber of moles

Choose one of the reactants and using its value calculate how many moles of the other are needed to react

Decide if you have more than this quantity of the secondreactant

For each of the two reactants calculate how many moles are present

8.51 g of magnesium powder was added to

100 cm3 of 2 mol l-1 hydrochloric acid.

Decide by calculation which of reactants is in excess

Mg + 2HCl MgCl2 + H2

Copy the equation and under the reactants write thenumber of moles

Mg + 2HCl MgCl2 + H2

1 mole 2 moles

We only need to consider the two reactants for this calculation

For the two reactants calculate how many moles are present

Mg Number of moles =mass

gram formula mass

HCl Number of moles = conc x volume in litres

Number of moles =8.51

24.3

Number of moles = 2 x 100/1000

= 0.35

Number of moles = 0.2

Choose one of the reactants and using its value calculate

how many moles of the other are needed to react

Mg + 2HCl MgCl2 + H2

1 mole 2 moles

Use the value of 0.35 moles of Mg

0.35 moles 0.7 moles

We need 0.7 moles for all of the Magnesium to react

Do you have more than this quantity of the second reactant ?

We need 0.7 moles of HCl

From the calculation

We have 0.2 moles of HCl

From the equation

We do not have enough HCl, we have too much Mg

Mg is in Excess

6.3 g of Magnesium reacts with 200 cm3 of 1 mol l-1 H2SO4.

Calculate the reactant which is in excessMg + H2SO4 MgSO4 + H2

1 mole + 1 mole

Mg No. of moles = 6.3/24.3 = 0.26

H2SO4 No. of moles = 1 x 200/1000 = 0.2

0.26 + 0.26

Need 0.26 moles of H2SO4 to react will all MgHave only 0.2 mole of H2SO4Magnesium is in excess

For Mg No. of moles = Mass / Gram Formula Mass

For H2SO4 No. of moles = concentration x Volume in litres