Question

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.

Answer 1

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`