A compound is found to be 43.7% phosphorus and 56.3% oxygen. Given the molecular mass as 285 ± 10, calculate the empirical and molecular formulas?

Question

Quizplus · Accepted Answer

To find the empirical formula, we need to determine the smallest whole number ratio of atoms present in the compound.

Assume we have 100 g of the compound. This means we have 43.7 g of phosphorus and 56.3 g of oxygen.

We can convert these masses to moles using the molar mass of each element (31 g/mol for P and 16 g/mol for O):

Phosphorus: 43.7 g / 31 g/mol = 1.41 moles
Oxygen: 56.3 g / 16 g/mol = 3.52 moles

We can then divide each mole value by the smallest mole value to get the mole ratio:

Phosphorus: 1.41 / 1.41 = 1
Oxygen: 3.52 / 1.41 = 2.5

The empirical formula is therefore P1O2.5, which we can simplify to PS1U1B12S1U1B0OS1U1B15S1U1B0 (multiply by 2 to get rid of the decimal and use subscripts to indicate the ratio of atoms).

To find the molecular formula, we need to know the molar mass of the compound. We are given a molecular mass of 285 ± 10, so the range of possible molecular masses is 275-295.

We can calculate the molecular formula by dividing the given molecular mass by the empirical formula mass:

Empirical formula mass: 31 + (2.5 x 16) = 71
Molecular formula mass range: 275-295

We can then divide the upper and lower ends of the molecular mass range by the empirical formula mass to get the range of possible molecular formula ratios:

Lower range: 275 / 71 = 3.87, which we can round to 4
Upper range: 295 / 71 = 4.15, which we can round to 4

This means the molecular formula is 4 times the empirical formula, giving us PS1U1B14S1U1B0OS1U1B60S1U1B0. We can simplify this to PS1U1B14S1U1B0OS1U1B110S1U1B0 by multiplying by 2 to get rid of the decimal and using subscripts to indicate the ratio of atoms.

A Compound Is Found to Be 43