Question

explain why the molar ratio is needed to determine the theoretical mass ratios

Answer 1

Is is related to stoichiometry of a reaction.

Let us solve it by an example:

$6CO_2+6H_2O\to C_6H_{12}O_6+6O_2$

Molar ratio is the proportion of reactants used which can be derived from stoichiometry of a reaction.

In this case ,molar ratio is $6:6:1$ for $CO_2:H_2O:glucose$

Hence by determining the molar ratio,we can determine the mass of glucose to be formed by using the proportion.

And the theoretical determined mass of glucose is theoretical mass of a product in the reaction by multiplying the ratio with their molar mass.