How do you calculate the mole ratio for two compounds to predict a base amount to neutralize an acid?

A sample of phosphoric acid, H3PO4, with a concentration of 1.0 M is being tested. 10 mL of the acid was measured in a 100 mL graduated cylinder, and a drop of phenolphthalein was added. Predict how much 1.0 M NaOH base will be added to just neutralize the acid (when the indicator just turns pink).

the response is asked to be made in mL.

1 Answer
tcw1
Feb 8, 2018

30 mL #NaOH#

Explanation:

First, write the reaction:

#H_3PO_4 + NaOH -> H_2O + Na_3PO_4#

Then, balance it:

#H_3PO_4 + 3NaOH -> 3H_2O + Na_3PO_4#

From these coefficients, the ratio can be calculated. Phenolphthalein turns pink at the equivalence point, meaning that there is an equal amount of #H^+# and #OH^-#. This is at a ratio of 3 mol #NaOH# and 1 mol #H_3PO_4#.

1.0 mol/L #H_3PO_4#* 0.010 L= 0.010 mol #H_3PO_4#

Multiplying this by the ratio (3) gives the number of moles of #NaOH#

0.010*3=0.030 mol #NaOH#

Converting back to moles:

0.030 mol #NaOH#/ x L=1.00 M #NaOH#
0.030 L #NaOH#= 30 mL #NaOH#

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