Assumption of 100g

[Samantha Man 1L]

Why is it that sometimes when they give you the percent composition of each element and the molar mass, you don't assume 100 g but instead you multiply that percentage by the molar mass to find the g of each element? I thought the molar mass is only supposed to be used when solving for the molecular formula?

[Reva Kakaria 1J]

Multiplying the percentage by the molar mass is more accurate and faster than assuming 100g and converting that into moles, and then dividing by the smallest number. You can then use the grams found by multiplying percentage by molar mass to convert into moles, which gives you the molecular formula, like you said. However, you can also use the molecular formula to find the empirical formula by dividing the subscripts of each element by a common factor.

[Mayaal_Khan_4H]

Sometimes the percentages don't add up to a 100 so you have to multiply the mass of each element by the molar mass so that the mass of all elects equal 1 to 100.