Some Basic Concepts of Chemistry — Practice

Question 1

Calculate the mass of 0.5 moles of CaCO3 (Ca = 40, C = 12, O = 16).

Accepted Answer

Molar mass = 40 + 12 + 48 = 100 g/mol. Mass = n x M = 0.5 x 100 = 50 g.

Question 2

A compound contains 92.3% C and 7.7% H by mass. Find its empirical formula. If its vapour density is 13, find the molecular formula.

Accepted Answer

Moles: C = 92.3/12 = 7.69, H = 7.7/1 = 7.7. Ratio C:H = 1:1, so EF = CH. Molar mass = 2 x VD = 26. n = 26/13 = 2. Molecular formula = C2H2 (acetylene).

Question 3

How many grams of O2 are needed to completely burn 24 g of CH4?

Accepted Answer

CH4 + 2O2 -> CO2 + 2H2O. n(CH4) = 24/16 = 1.5 mol. n(O2) = 2 x 1.5 = 3 mol. Mass O2 = 3 x 32 = 96 g.

Question 4

2 g H2 reacts with 16 g O2 to form water. Find the limiting reagent and the mass of water formed.

Accepted Answer

2H2 + O2 -> 2H2O. n(H2) = 1 mol, n(O2) = 0.5 mol. 0.5 mol O2 needs 1 mol H2; exactly 1 mol H2 is available, so both are consumed. Water = 1 mol = 18 g.

Question 5

Why is molarity temperature-dependent but molality is not?

Accepted Answer

Molarity is defined using the volume of solution, which expands or contracts with temperature. Molality is defined using the mass of solvent, which does not change with temperature.

Practice — Some Basic Concepts of Chemistry

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