6.023 × 10²² molecules are present in 10 g of a substance 'x'. The molarity of a solution containing 5 g of substance 'x' in 2 L solution is _____ × 10⁻³

Solution

Chemistry ma’am · 3 steps

First find the molar mass from the molecule count, then molarity is just moles over litres.

1. Moles in 10 g. One mole has 6.023 × 10²³ molecules. So 6.023 × 10²² molecules is (6.023 × 10²²)/(6.023 × 10²³) = 0.1 mol.
2. Molar mass. 10 g of x is 0.1 mol, so the molar mass of x is 10/0.1 = 100 g mol⁻¹.

   M = n/V = (5/100)/2 = (0.05 mol)/(2 L) = 0.025 mol L⁻¹

3. So. The molarity is 0.025 M = 25 × 10⁻³ M, so the answer is 25.

The common slip: 50. Forgetting to divide the 0.05 mol by the 2 L volume gives 50, not 25.

The idea to keep. A molecule count tells you moles, and moles with a mass tell you the molar mass. Once you have that, molarity is moles of solute divided by litres of solution.

Answer from the NTA final answer key. Solution by PrepWiser.