Answer to Question #170496 in General Chemistry for Leila Marie

Question

H3PO4 (aq) ⇌ H + (aq) + H2PO4 - (aq)      Ka = 7.2 x 10 -3

Determine the shift of the equilibrium if 100 ml of 0.005M H3PO4 is added to the original solution.

Answer 1

Equilibrium won't shift

H₃PO₄ (aq) $\leftrightarrow$ H⁺ (aq) + H₂PO$_4^-$ (aq)

1 moles of H₃PO₄ gives 1 mole of H⁺ and 1 mole of H₂PO$_4^-$

no. Of moles = molarity × volume

No. Of moles = 0.005M×0.100L

No. Of moles = 0.0005

0.0005moles of H₃PO₄ gives 0.0005 mole of H⁺ and 0.0005 mole of H₂PO$_4^-$

Because there is an equal number of moles on both sides of the reaction, an increase in volume will have no effect on the equilibrium and thus there is no shift in the direction.