Question

An unknown compound has a percent composition of 75.95% C, 17.72% N, and 6.33% H by mass with a molar mass of about 240 g/mol. Determine the molecular formula of the dye. (Atomic mass of C-12.011 g/mol; N =14.007 g/mol; H- 1.008 g/mol).

Answer 1

To determine the molecular formula, we first need to determine the empirical formula.

1. Assume we have 100 g of the compound, which means we have 75.95 g C, 17.72 g N, and 6.33 g H.

2. Convert the mass of each element to moles using its molar mass:

- Moles of C = 75.95 g / 12.011 g/mol = 6.316 mol
- Moles of N = 17.72 g / 14.007 g/mol = 1.264 mol
- Moles of H = 6.33 g / 1.008 g/mol = 6.286 mol

3. Divide each mole value by the smallest mole value to get the mole ratio:

- C:N:H = 6.316 mol / 1.264 mol / 6.286 mol = 5 : 1 : 5

4. The empirical formula is C5H5N.

5. To determine the molecular formula, we need to know the molecular mass. The molar mass of the compound is given as 240 g/mol, which is approximately 10 times the molar mass of the empirical formula (5[C5H5N] = 225 g/mol).

6. Divide the molar mass of the compound by the molar mass of the empirical formula to get the scaling factor:

- Scaling factor = 240 g/mol / 225 g/mol ≈ 1.07

7. Multiply the subscripts in the empirical formula by the scaling factor to get the molecular formula:

- C5H5N becomes C5.36H5.36N, which we round to C5.4H5.4N.

Therefore, the molecular formula of the dye is likely C5.4H5.4N.