Why do you need to use amount in moles to
solve stoichiometry problems? Why can’t
you just convert from mass to mass?
Chemicals don't react in grams. They react in moles. For example, the equation
2NaOH + H2SO4 ==> Na2SO4 + 2H2O
tells you that 2 moles NaOH react with 1 mole H2SO4 to produce 1 mole Na2SO4 and 2 moles H2O. If we start with grams NaOH and want to know how many grams H2SO4 will be needed, we first convert grams NaOH to moles, convert moles NaOH to moles H2SO4 using the coefficients in the equation, then convert moles H2SO4 to grams.