How many molecules contain a) 1.5 mol of hydrogen b) 7 g of hydrogen?

a) Given:
n (H2) = 1.5 mol.
To find:
N (H2) -?
Solution:
N (H2) = Na × n = 6 × 10²³ × 1.5 mol = 9 × 10²³ molecules.
Answer: 9 × 10²³ molecules.

b) Given:
m (H2) = 7 g.
To find:
N (H2) -?
Solution:
M (H2) = 2 g / mol.
n (H2) = m / M = 7 g: 2 g / mol = 3.5 mol.
N (H2) = Na × n = 6 × 10²³ × 3.5 mol = 21 × 10²³ molecules.
Answer: 21 × 10²³ molecules.