Question

Find the number of molecules in 90 g of water. Molecular weight of water is 18 g per mole

Answer 1

Number of molecules =
$\frac{given \: mass \: }{molar \: mass \: } \times avogadro \: number$
Molar mass Of H2 O =

H = 1u
O = 16u
H2O = 1 ×2 +16 = 18u
Molar mass = 18g



Number =
$\frac{90}{18} \times 6.022 \times {10}^{23} \\ = 5 \times 6.022 \times {10}^{23} \\ = 30.11 \times {10}^{23} \\ = 3.011 \times {10}^{24}$

hope it helps