3 KOH + H3PO4 → K3PO4 + 3 H2O The chemical reaction above produced 17.89 moles of water. How many moles of H3PO4 were used?

Answer 1

Since the water:H3PO4 molar ratio from the balanced eq’n is 3:1, then whatever water is, you can divide by 3 to get H3PO4. But a better way to see it is with unit analysis method. Start with the given and multiply by the 3:1 molar ratio with the units set up to cancel to produce the units you want for an answer. Something like this:

17.89 mol H2O x 1 mol H3PO4/3 mol H2O

The units of mol H2O cancel leaving units of mol H3PO4 as the units of the answer. Also note that as predicted, the answer is 17.89/3. Hope that helps.