7. What is the mass fraction (%) of oxygen in aluminum oxide $\mathrm{Al}_{2} \mathrm{O}_{3}$?
a) $53 \%$

b) $27 \%$

c) $16 \%$

d) $102 \%$
7. $53 \% . \operatorname{Mr}(\mathrm{Al} 2 \mathrm{O} 3)=27 * 2+16 * 3=102 \quad \mathrm{~W}(\mathrm{Al})=27 * 2 / 102 * 100 \%=53 \%$ 

7. $53 \% . \operatorname{Mr}(\mathrm{Al} 2 \mathrm{O} 3)=27 * 2+16 * 3=102 \quad \mathrm{~W}(\mathrm{Al})=27 * 2 / 102 * 100 \%=53 \%$

The text is already in a format that is a mix of mathematical notation and chemical formula, which is the same in English. Therefore, the translation is identical to the original text.