113. the elemental mass percent composition of ibuprofen (an aspirin substitute) is 75.69% c, 8.80% h, and…

Explanation:

Step1: Assume 100g of ibuprofen

Assume we have 100g of the compound. Then the masses of C, H, and O are 75.69g, 8.80g, and 15.51g respectively.

Step2: Calculate the moles of each element

The molar - mass of C is 12.01g/mol, of H is 1.01g/mol, and of O is 16.00g/mol. The moles of C: $n_{C}=\frac{75.69g}{12.01g/mol}\approx6.302mol$ The moles of H: $n_{H}=\frac{8.80g}{1.01g/mol}\approx8.713mol$ The moles of O: $n_{O}=\frac{15.51g}{16.00g/mol}\approx0.969mol$

Step3: Find the mole - ratio of the elements

Divide each number of moles by the smallest number of moles (0.969mol). For C: $\frac{6.302mol}{0.969mol}\approx6.5$ For H: $\frac{8.713mol}{0.969mol}\approx9$ For O: $\frac{0.969mol}{0.969mol}=1$ Since we need whole - number ratios, multiply each ratio by 2 to get whole numbers. For C: $6.5\times2 = 13$ For H: $9\times2=18$ For O: $1\times2 = 2$

Answer:

$C_{13}H_{18}O_{2}$