Step 1
The reaction:
H3PO4 (aq) + 2 KOH (aq) => K2HPO4 (aq) + 2 H2O (l)
The equation must be always completed and balanced.
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Step 2
Information provided:
H3PO4) 40.00 mL of 0.0656 M
40.00 mL x (1 L/1000 mL) = 0.0400 L
KOH) 0.1500 M
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Step 3
The number of moles of H3PO4:
Molarity (M or mol/L) = moles/volume (L)
Molarity x volume (L) = moles
0.0656 mol/L x 0.0400 L = 2.62x10^-3 moles H3PO4
The number of moles of H3PO4 = 2.62x10^-3 moles
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Step 4
The number of moles of KOH: by stoichiometry,
H3PO4 (aq) + 2 KOH (aq) => K2HPO4 (aq) + 2 H2O (l)
1 mole of H3PO4 ------------ 2 moles KOH
2.62x10^-3 moles H3PO4 ------------ X
X = 2.62x10^-3 moles H3PO4 x 2 moles KOH/1 mole of H3PO4
X = 5.24x10^-3 moles KOH
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Step 5
The volume of KOH:
Molarity = moles/volume (L)
Volume (L) = moles/molarity
Volume (L) = 5.24x10^-3 moles/0.1500 mol/L
Volume = 0.0349 L x (1000 mL/1 L) = 34.9 mL
Answer: 34.9 mL
