What is the pH of a mixture of $ 1.0 \; L \ NaOH \; 0.10 \; \frac{mol}{L} $ with $ 1.0 \; L \; H_3PO_4 \; 0.10 \; \frac{mol}{L} $? Write the (nonionic) equation of the reaction !
For answers, use (possibly several times) the arrows ↑ Down! and ↓ Up! Complete please this question before moving on to the next one!
$NaOH+H_3PO_4 \rightarrow NaH_2PO_4+H_2O$
Complete the following table:
| $NaOH$ | + | $H_3PO_4$ |  |
$NaH_2PO_4$ | + | $H_2O$ | |
| initial nb. of moles | 0.10 | 0.10 | 0 | ||||
| variation of mole nbs | ........ | ........ | ........ | ||||
| final nb. of moles | ........ | ........ | ........ |
| $NaOH$ | + | $H_3PO_4$ | |
$NaH_2PO_4$ | + | $H_2O$ | |
| initial nb. of moles | 0.10 | 0.10 | 0 | ||||
| variation of mole nbs | -0.10 | -0.10 | +0.10 | ||||
| final nb. of moles | 0 | 0 | 0.10 |
Write the solutes present at the end with their molarities !
$[NaH_2PO_4]=\frac{0.10}{2.0}=0.05\frac{mol}{L}$
Analyse the final mixture!
$NaH_2PO_4$ dissociates to $H_2PO_4^-$ and $Na^+$ $Na^+$ has no influence. $H_2PO_4^-$ is an ampholyte!
Calculate its pH!
$pH$ $=$ $\frac{1}{2}pK_{a1}+\frac{1}{2}pK_{a2}$ $=$ $\frac{1}{2}2.12+\frac{1}{2}7.21$ $=$ $4.67$ .
