How to calculate concentration of acids and alkalis?

How to calculate concentration of acids and alkalis?

Concentrations of Acids and Alkalis

1. A solution is a mixture formed by dissolving a solute in a solvent.
   Solute + solvent → solution
   For example, a sugar solution is prepared by dissolving sugar (solute) in water (solvent).
2. By dissolving varying amounts of sugar in a fixed volume of water, sugar solutions of different concentrations are obtained.
3. Hence, the concentration of a solution refers to the quantity of solute in a given volume of solution which is usually 1 dm³ of solution.
4. The quantity of solute can be measured in grams or moles.
5. Therefore, concentration of a solution can be expressed in two ways:
   (a) mass of solute in grams in 1 dm³ of solution
   (b) number of moles of solute in 1 dm³ of solution
6. The concentration expressed in moles per dm³ is called molarity or molar concentration.
   Molarity is the number of moles of solute present in 1 dm³ of solution.
   
7. Figure shows the two concentration units for a solution.
   

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Calculating concentration in g dm^-3

1. 4.0 g of sodium hydroxide, NaOH is dissolved in enough water to make 200 cm³ of solution.
What is the concentration of the solution in g per dm³?
Solution:

2. A 400 cm³ salt solution was evaporated to dryness and 18.0 g of sodium chloride, NaCl, was left behind. Calculate the concentration of the salt solution in g dm^-3.
Solution:

3. Concentrated phosphoric acid, H₃PO₄ was added slowly to water to produce 5.0 dm³ of diluted acid solution. 100 g of pure acid was present in the solution. What is the concentration of the acid in g dm^-3.
Solution:

Calculating concentration in mol dm^-3

1. What is the molarity of a solution prepared by dissolving 0.15 mole of ammonia in enough distilled water to make 250 cm³ of solution?
Solution:

2. Copper(II) oxide reacts with dilute sulphuric acid to produce 1.2 moles of copper(II) sulphate, CuSO₄. The final volume of the salt solution is 1.8 dm³. What is the molar concentration of the salt solution?
Solution:

Conversion of units of concentration

The equations below show the interconversion between the two units of concentrations:

1. A 200 cm³ solution was found to contain 24.5 g of dissolved sulphuric acid. What is the molarity of the acid solution?
[Relative atomic mass: H, 1; O, 16; S, 32]
Solution:


2. An alkaline solution of potassium hydroxide, KOH has a concentration of 2.8 g dm^-3. Calculate the molarity of the solution.
[Relative atomic mass: H, 1; O, 16; K, 39]
Solution:

Calculating the amount of solute

The number of moles of a solute in a given volume of solution can be calculated using the following equation.

1. Calculate the mass of hydrochloric acid in 300 cm³ of 2.0 mol dm^-3 hydrochloric acid.
[Relative atomic mass: H, 1; Cl, 35.5]
Solution:

2. A student pipetted 25.0 cm³ of sodium hydroxide solution into a conical flask. The concentration of the alkali is 1.20 mol dm^-3. Calculate the number of moles of sodium hydroxide in the flask.
Solution:

3. Calculate the number of moles of hydrogen ions in 500 cm³ of 1.0 mol dm³ sulphuric acid.
Solution: