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Calculation of excess

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

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Calculation of excess

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  1. Calculation of excess In an excess calculation you will be given the quantities of two reactants

  2. We can tackle excess calculations in four steps Copy the equation and under the reactants write the number of moles For each of the two reactants calculate how many moles are present 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 second reactant

  3. Mg + 2HCl MgCl2 + H2 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

  4. Mg + 2HCl MgCl2 + H2 Copy the equation and under the reactants write the number of moles 1 mole 2 moles We only need to consider the two reactants for this calculation

  5. mass Number of moles = gram formula mass 8.51 Number of moles = 24.3 For the two reactants calculate how many moles are present Mg = 0.35 Number of moles = conc x volume in litres HCl Number of moles = 2 x 100/1000 Number of moles = 0.2

  6. Mg + 2HCl MgCl2 + H2 1 mole 2 moles Choose one of the reactants and using its value calculate how many moles of the other are needed to react 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

  7. Do you have more than this quantity of the second reactant ? From the equation We need 0.7 moles of HCl From the calculation We have 0.2 moles of HCl We do not have enough HCl, we have too much Mg Mg is in Excess

  8. Mg + H2SO4 MgSO4 + H2 6.3 g of Magnesium reacts with 200 cm3 of 1 mol l-1 H2SO4. Calculate the reactant which is in excess 1 mole + 1 mole 0.26 + 0.26 Mg No. of moles = 6.3/24.3 = 0.26 For Mg No. of moles = Mass / Gram Formula Mass H2SO4 No. of moles = 1 x 200/1000 = 0.2 For H2SO4 No. of moles = concentration x Volume in litres Need 0.26 moles of H2SO4 to react will all Mg Have only 0.2 mole of H2SO4 Magnesium is in excess

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