Sodium hydroxide reacts with phosphoric(V) acid according to the equation: 3NaOH + H3PO4 -> Na3PO4 + 3H2O 25.00 cm3 of 0.10 mol dm-3 sodium hydroxide reacts with 0.05 mol dm-3 H3PO4. The volume of H3PO4, in cm3, required for neutralisation is?
This is a simple moles equation and the method with any moles equation is find the number of moles of something, use the chemical equation to work out the number of moles of the thing you need to find and the convert this value into what you need e.g. the mass or volume of the compound.To start, we only have enough information to work out the number of moles of NaOH so let's do this! First, divide 25/1000 = 0.025 to convert the cm3 NaOH volume to dm3. We do this because the units of everything else is in dm3 rather than cm3. To work out the number of moles of NaOH we then multiply 0.025 by 0.1 to get 2.5x10-3mol of NaOH. Then, using the chemical equation we divide the number of moles of NaOH by 3 as this is the ratio of NaOH to H3PO4 in the equation. This gives us 1/1200 mol. To find out what volume of H3PO4 we require to get this amount of moles of H3PO4 we then divide the number of moles by the concentration of the H3PO4 solution (0.05mol/dm3). This gives us 1/60 dm3 for volume. However, we are not done yet! The question asks for the answer in cm3 so we have to convert the value we have for volume into cm3 by multiplying by 1000. This gives us 1/60 or 16.67 cm3.
