# Calculation Using Chemical Equations (Quantitative Chemistry)- slide presentation ppt

Calculations using Chemical Equations

IGCSE 1

Mr. Markus

Calculations using chemical equations

A     +    B    🡪    C    +   D

reactants products

ex: 2Na(s)  +  Cl2(g)  🡪  2NaCl(s)

CH4(g)  +  2O2(g)  🡪  CO2(g)  +  2H2O(g)

• A balanced chemical equation provides quantitative information about the reactants and products

• It allows us to calculate the masses or volumes of the reactants and products

When carrying out calculations from equations, follow these steps:

STEP 1 : Write the balanced equation for the reaction

STEP 2 : Write the mole ratio for the chemical substances in question

STEP 3 : Calculate number of mole of each of the substance in question

STEP 4 :Find the quantity required by equating the values from step 2 and step 3

example

• Common salt sodium chloride is prepared by reacting sodium with chlorine. What mass of sodium chloride is produced from 1.42 g of chlorine?

Solutions:

STEP1: (equation) 2Na(s)  +  Cl2(g)  🡪  2NaCl(s)

STEP2: (mole ratio) moles of NaCl =

moles of Cl2      1

STEP3: (determine number of moles)

number of moles of Cl2

= mass/Mr

= 1.42 g/ 2(35.5) g/mol

= 0.02 mol

moles of NaCl =

0.02               1

moles of NaCl          = 2 x 0.02 = 0.04 mol

STEP4 : mass of NaCl = 0.04 mol x Mr of NaCl

= 0.04 mol x (23+35.5) g/mol

= 2.34 g

Limiting reactants

• The limiting reactant or limiting reagent is the chemical that is completely consumed in a reaction

• The reaction stops as soon as the limiting reactant is totally consumed

• The limiting reactant limits the amount of product formed

Practice

1)  Propane, C3H8, burns in excess oxygen to produce carbon dioxide and water. How many moles of oxygen required to burn 3.0 g of C3H8? And what is the mass of water produced?

Practice

2)   3.50 moles of sulphur dioxide were allowed to react with 1.10 moles of oxygen to form sulphur trioxide. What is the limiting reactant? How many grams of sulphur trioxide formed?

Practice

3)   When a solution containing 3.28 g of Na3PO4 is mixed with a solution containing 6.62 g of Pb(NO3)2, a double decomposition takes place to produce Pb3(PO4)2 and NaNO3 . Calculate the mass of lead(II) phosphate formed.

Practice

4)  100 cm3 of 0.50 mol dm-3 aqueous silver nitrate was mixed with 100 cm3 of 0.20 mol. dm-3 aqueous potassium chromate (VI). The equation for the reaction is:

K2CrO4(aq) + 2AgNO3(aq) 🡪  Ag2CrO4(s)  +  2KNO3(aq)

(a) what is the number of moles of silver nitrate used?

(b) what is the number of moles of K2CrO4 used?

(c) which is the limiting reactant in this reaction

(d) calculate the mass of Ag2CrO4 precipitated