A and then C
Dimensional analysis
[tex]\begin{array}{lll}n & = & m / M \\ & = & 8.33 \; \text{g H} / (1.008 \; \text{g H} \cdot\text{mol H}^{-1})\\ & = & 8.26 \; \text{mol H}\end{array}[/tex]
[tex]\begin{array}{lll} N & = & n \cdot A_r \\ & = & 8.26 \; \text{mol H} \times 6.023 \times 10^{23} \; \text{mol}^{-1} \\ &= & 4.93 \times 10^{24} \; \text{H atoms}\end{array}[/tex]
[tex]\begin{array}{lll}N & = & n \cdot A_r \\ & = & 0.417 \; \text{mol O}\times 6.02 \times 10^{23} \; \text{mol}^{-1} \\ & = &2.51 \times 10^{23} \; \text{O atoms}\end{array}[/tex]
