# 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.

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tcw1 Share
Feb 8, 2018

30 mL $N a O H$

#### Explanation:

First, write the reaction:

${H}_{3} P {O}_{4} + N a O H \to {H}_{2} O + N {a}_{3} P {O}_{4}$

Then, balance it:

${H}_{3} P {O}_{4} + 3 N a O H \to 3 {H}_{2} O + N {a}_{3} P {O}_{4}$

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 $O {H}^{-}$. This is at a ratio of 3 mol $N a O H$ and 1 mol ${H}_{3} P {O}_{4}$.

1.0 mol/L ${H}_{3} P {O}_{4}$* 0.010 L= 0.010 mol ${H}_{3} P {O}_{4}$

Multiplying this by the ratio (3) gives the number of moles of $N a O H$

0.010*3=0.030 mol $N a O H$

Converting back to moles:

0.030 mol $N a O H$/ x L=1.00 M $N a O H$
0.030 L $N a O H$= 30 mL $N a O H$

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