Question

Phenyl magnesium bromide is used as a Grignard in organic synthesis. determine it’s empirical formula and molecular formula of this compound ? the molar mass of the compound is 181.33g/mol and it contains 39.7458%g/mol C, 2.77956% H, 13.4050% Mg and 44.0697% Br

Answer 1

For this question we will find the empirical formula and the molecular formula for a compound made of 39.7458% of C, 2.77956% of H, 13.4050% of Mg, and 44.0697% of Br. The mass we are calculating will be 181.33g/mol. The first thing we need to do is to find out how many grams of each element we actually have:

181.33g/mol = 100%

x grams = 39.7458%

x = 72.07 grams of C

181.33 = 100%

x grams = 2.77956%

x = 5.04 grams of H

181.33 = 100%

x grams = 13.4050%

x = 24.307 grams of Mg

181.33 = 100%

x grams = 44.0697%

x = 79.91 grams of Br

Now we have the mass for each element, we need to find the number of moles that each element has based on their masses and the molar mass of each one:

Carbon = 12g/mol

H = 1g/mol

Mg = 24.305g/mol

Br = 79.9g/mol

For Carbon:

12g = 1 mol

72.07g = x moles

x = 6 moles of C

For Hydrogen:

1g = 1 mol

5.04g = x moles

x = 5 moles of H

For Magnesium:

24.305g = 1 mol

24.307g = x moles

x = 1 mol of Mg

For Bromide:

79.90g = 1 mol

79.91g = x moles

x = 1 mol of Br

The molecular formula for this compound will be C6H5MgBr, this is Phenylmagnesium bromide, this will also be its empirical formula, since there is no way we can divide the numbers and make an empirical formula.