Calculate the pH of 0.01 M NaOH solution. Explain how pH matters in our body, in farming and in the environment.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Write the formulas. pOH = −log₁₀[OH⁻] and pH = 14 − pOH.
2 · Work out the pH.
[OH⁻] = 0.01 = 10⁻² mol L⁻¹
pOH = −log(10⁻²) = 2
pH = 14 − 2 = 12 (basic)
3 · Digestion. The stomach makes hydrochloric acid (pH about 2) to digest food. Excess acid is neutralised by antacids such as Mg(OH)₂.
4 · Blood. Blood pH is kept between 7.35 and 7.45; a large change is harmful.
5 · Teeth. Below pH 5.5 tooth enamel dissolves. Basic toothpaste neutralises the acid made by bacteria from sugar.
6 · Soil and rain. Each crop needs a suitable soil pH, and acidic soil is treated with lime. Acid rain, made acidic by gases such as sulphur dioxide, damages plants and buildings.
7 · Draw a pH scale. [A line from 0 to 14 marking stomach juice (about 2), tooth decay line (5.5), pure water (7) and blood (7.4)]
8 · Close in one line. Therefore 0.01 M NaOH has pH 12, and keeping the right pH is vital for our body, crops and environment.
Scoring tip: For a base, always find pOH first. In the importance part, give a pH value with each example.
