The pH value of a sample of multiple-distilled water is
- A.zero
- B.14
- C.very near to zero
- D.very near to seven
Correct answer
Explanation
The correct answer is D, very near to seven. Pure water is neutral, and a neutral solution has a pH of 7 at 25 °C.
pH is the negative logarithm of the hydrogen ion concentration. In pure water a tiny fraction of molecules split into H⁺ and OH⁻ ions in equal numbers, each about 10⁻⁷ mol per litre at 25 °C, which gives pH 7. Repeated distillation removes dissolved salts, so the sample comes very close to pure water. In practice its pH reads slightly below 7, because carbon dioxide from the air dissolves to form weak carbonic acid, and the neutral point itself shifts a little with temperature; that is why the answer is near seven, not exactly seven.
A is wrong because pH zero means a very strong acid, about 1 mol per litre of H⁺ ions.
B is wrong because pH 14 means a very strong base, such as a concentrated sodium hydroxide solution.
C is wrong because a pH near zero is strongly acidic, which pure water is not.
Exam tip: below 7 acidic, 7 neutral, above 7 basic; each step of one pH unit is a tenfold change.
Practice Questions
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- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
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- C.Urea is produced from digestion of starch.
- D.Urea is produced in lung and kidney.
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- B.Objective lens-Condenser-Body tube-Eyepiece
- C.Condenser-Objective lens-Body tube-Eyepiece
- D.Eyepiece-Objective lens-Body tube-Mirror
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- D.Haemoglobin is only used for blood clotting and not for carrying gases.
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- B.Histones are proteins that are present in nucleus in association with DNA.
- C.Histones are proteins associated with lipids in the cytosol.
- D.Histones are proteins associated with carbohydrates in the cytosol.
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