What is the percentage by mass of oxygen in $NaOH$? (Atomic mass of $Na=23 \ u, O=16 \ u, H=1 \ u$)

Which of the following is the formula for calcium chlorite?

Hemoglobin contains $0.34 \%$ of iron by mass. The number of $Fe$ atoms in $3.3 \ g$ of hemoglobin is. (Given : Atomic mass of $Fe$ is $56 \ u, N_A = 6.022 \times 10^{23} \ mol^{-1}$)

The sulphate of a metal $M$ contains $9.87\%$ of $M$. This sulphate is isomorphous with $ZnSO_4 \cdot 7H_2O$. The atomic weight of $M$ is (in $.3$)

$0.0833 \, mol$ of a carbohydrate with the empirical formula $CH_2O$ contains $1 \, g$ of hydrogen. What is the molecular formula of the carbohydrate?

Hemoglobin contains $0.334 \% \, Fe$ by weight. If the molecular mass of hemoglobin is approximately $67200 \, g/mol$,determine the number of iron atoms in one molecule of hemoglobin. (Atomic mass of $Fe = 56 \, g/mol$)