Question

What mass of nitrogen, N2, will contain the same number of molecules as 1.8 g of water, H2O ?

(Atomic masses : N = 14 u ; H = 1 u ; O = 16 u)

Answer 1

Mass of water = 1.8g

We know that equal moles of all substance contain equal number of molecules (1 mole of all substances contains 6.022 × 10²³ molecules).
The first step will be to convert 1.8g  water into moles.

1 mole of water = (2 × 1) + (1 × 16) = 18g

18g water = 1 mole

1.8g  water = 0.1 moles

Since, equal moles of all substances contain equal number of molecules, 0.1 moles of water will have the same number of molecules as present in 0.1 moles of nitrogen.

The next step will be to find the mass of 0.1 moles of nitrogen in grams.

1 mole of nitrogen = 28g

0.1 moles of nitrogen = 28 × 0.1 = 2.8g

Hence, 2.8g  nitrogen will contain the same number of molecules as present in 1.8g  water.