NumericalModerate2 marks
Calculate the number of moles present in 160 gm of NaOH. [Atomic Mass: Na = 23, H= 1, O= 16]
Topic: Mole concept, gas volumes and stoichiometry
Previous-year question practice database
Answer
Exam answer
Given: mass of NaOH = 160 g. [Na = 23, O = 16, H = 1]
Molar mass of NaOH = 23 + 16 + 1 = 40 g mol⁻¹
Moles = mass ÷ molar mass = 160 ÷ 40 = 4 mol
Number of moles = 4 mol
Explanation
Add the three atomic masses for the molar mass, then divide. The only real trap is arithmetic — 23 + 16 + 1 = 40, not 39 or 41.
Note that NaOH is ionic, so strictly this is a FORMULA mass rather than a molecular mass, and 4 moles of NaOH contains 4 moles of Na⁺ and 4 moles of OH⁻. The calculation is identical either way.
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