L8: Solution Stoichiometry

Objectives

Perform stoichiometric calculations using concentrations of solutions expressed in g/dm³ and mol/dm³.

A reminder to please only use your IGCSE periodic table for stoichiometry.

Molar concentration of solutions

Recall that solutions consist of a solute (e.g. sugar) dissolved in a solvent (e.g. water). Concentration is the amount of solute dissolved in a given volume of solvent.

Up to now, you have usually expressed concentration in units like grams per litre (g/L). This is the mass concentration. Because we use moles and molar ratios in stoichiometry, it is much more useful to express concentration using moles. Recall that one litre is equal to one cubic decimetre (dm³).

We express concentration in moles per cubic decimetre (mol/dm³). This is known as molar concentration or molarity.

Calculating moles from the concentration of a solution

You must memorize this triangle. It is quicker with the symbols if you can.

number of moles, n (mol)
molar concentration, also called Molarity, C (mol/dm³)
volume, V (dm³)

Solution Stoichiometry

Some useful examples if you missed the lesson

Calculating molar concentration: Example

What is the molar concentration/molarity of a solution of sodium hydroxide (NaOH), that contains 10g of NaOH and a volume of 250cm³?

Find how many moles of NaOH are present.

Convert volume to dm³ and find the concentration in mol/dm³

Answer

Find how many moles of NaOH are present.

M_r(NaOH) = 23 + 16 + 1 = 40 g/mol

n(NaOH) = m = _10g = 0.25 mol

M_r 40g/mol

Convert volume to dm³ and find the concentration in mol/dm³

250 dm³ = 0.250dm³

1000

c = n = 0.25mol = 1 mol/dm³

V 0.250dm³

Full worked example

20.0cm³ of HCl solution reacts fully with 25.0cm³ of 0.10mol/dm³ NaOH solution to form sodium chloride and water. What is the concentration of the HCl solution?

Write balanced symbol equation and the info you have/need.

Convert volumes to dm³

Find moles of NaOH

Use molar ratio to find moles of HCl

Calculate concentration of HCl

Answer

Write balanced symbol equation and the info you have/need.

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) (already balanced)

V=20.0cm³ V=25.0cm³

c = ? c=0.10mol/dm³

Convert volumes to dm³

V(HCl) = 20.0cm³ x 1dm³ = 0.02dm³

1000cm³

V(NaOH) = 25.0cm³ x 1dm³ = 0.025dm³

1000cm³

Find moles of NaOH

c = n → n = cV = 0.10 mol/dm³ x 0.025dm³ = 0.0025 mol

Use molar ratio to find moles of HCl

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

↓x0.0025 ↓x0.0025

0.0025HCl 0.0025NaOH

Calculate concentration of HCl

c = n = 0.0025mol = 0.125 mol/dm³

V 0.02dm³

ANSWER: the concentration of the HCL solution is 0.125 mol/dm³

Solution calculation

The video says, “In a titration…”. A titration is just a reaction between two solutions. You do not need to know more than that for now.

Practice Questions

Silver nitrate reacts with potassium chloride in the following balanced reaction:

AgNO₃ + KCl → AgCl + KNO₃

If 45cm³ of 1.5mol/dm³ AgNO₃ solution is reacted with excess KCl, how many grams of AgCl can be formed?

Answer

9.7g of AgCl

2. Calcium hydroxide reacts with hydrochloric acid as shown in this unbalanced equation:

Ca(OH)₂ + HCl → CaCl₂ + H₂O

1. Balance the equation
2. How many grams of calcium hydroxide are required to react with 350cm³ of 0.40mol/dm³HCl?

Answer

2a) Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O

2b) 5.2g of Ca(OH)₂

Stoichiometry L8: Solution Stoichiometry

Stoichiometry L8: Solution Stoichiometry MS

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