Question

How do you calculate the quantity in mol and grams of the reactants, when given the mass in mL of the product?

Answer 1

You would have done well to furnish an example AND do you know how to work a stoichiometry problem otherwise? Not much difference.
Example: Calculate the grams (and moles) of H2 needed to produce 100 mL of H2O when reacted with H2 gas @ STP. Assume density of H2O is 1.00g/mL.
..............2H2 + O2 ==> 2H2O
So you want 100 mL H2O. With a density of H2O of 1.00 g/mL that means you want 100 g. How many moles is that? That's mols = g/molar mass = 100 g x (1 mol/18 g) = 55.5
How many moles H2 on the reactant side is necessary to do that? That's
55.5 g H2O x (2 mols H2/2 moles H2O) = 55.5 mols H2 needed.
Now convert that to grams H2. g H2 = moles H2 x molar mass H2 = ?
If this isn't exactly what you wanted post something of your choice and I'll take another crack at it. Remember that are two ways to arrive at moles.
moles of a solid = grams/molar mass = ?
moles of a solution = M x L = molarity x liters = ?
If you have a solution with a density you convert to grams first (mass = volume x density then use mass/molar mass to get moles.