Answers to questions on pH regulation of the stomach

1. Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

NaHCO₃ + HCl → NaCl + CO₂ + H₂O

2. A pH of 2 means that the concentration of the hydrochloric acid is 0.0100 mol dm^-3
Amount of HCl in 10 cm³ = 10/1000 x 0.0100 = 1.00 x 10^-4 mol

CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O
1 mole of CaCO₃ neutralises 2 mol of HCl
Amount of CaCO₃ required = ½ x 1.00 x 10^-4 mol = 5.00 x 10^-5 mol
M_r of CaCO₃ = 40.08 + 12.01 + (3 x 16.00) = 100.09 g mol^-1
Hence mass of CaCO₃ required = 100.09 x 5.00 x 10^-5 = 0.0050g (5.0 mg)

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
Since 1 mol of Mg(OH)₂ neutralises 2 mol of HCl
Amount of Mg(OH)₂ required = 5.00 x 10^-5 mol (which is the same amount (in mol) as the calcium carbonate).
M_r of Mg(OH)₂ = 24.31 + (2 x 17.01) = 58.33 g mol^-1
Hence mass of Mg(OH)₂ required = 58.33 x 5.00 x 10^-5 = 0.0029g (2.9 mg)

Therefore less mass of Mg(OH)₂ required.

3. (a) Heartburn is a form of indigestion caused by acid from the stomach rising up into the esophagus. It often occurs in pregnant women as the hormones produced during pregnancy have a relaxing effect on the valve at the entrance to the stomach. This can be further exacerbated as the foetus grows in size in later pregnancy as the larger uterus puts more pressure onto the stomach.

(b) sodium alginate

4. Ranitidine acts as an H2-histamine receptor inhibitor in the cells of the gastric lining, which reduces the secretion of stomach acid.
Omeprazole reduces the amount of stomach acid by inhibiting the gastric proton pump enzyme, which is responsible for secreting H⁺ ions into gastric juice in the stomach.

5. (a) [CH₃COO⁻(aq)] = [CH₃COOH(aq)] so pH = pK_a = 4.76

(b) M_r of NaCH₃COO = 22.99 + (2 x 12.01) + (3 x 1.01) + (2 x 16.00) = 82.14 g mol^-1
0.500 g = 0.500/82.14 = 6.09 x 10^-3 mol
[CH₃COO⁻(aq)] = 6.09 x 10^-3 x 1000/50 = 0.122 mol dm^-3
pH = pK_a + log₁₀ (0.122/0.100) = 4.76 + log₁₀ 1.22 = 4.85

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