Question

A phosphate buffer solution is prepared by dissolving 25.0 g of potassium hydrogen phosphate (K$_2$HPO$_4$; FM = 174.18) and 12.0 g of potassium dihydrogen phosphate (KH$_2$PO$_4$; FM = 136.09) in 250. mL of the buffer solution. (a) Calculate the molar concentrations of HPO$_4$$^{2-}$ and H$_2$PO$_4$$^-$, respectively, and the pH of the buffer solution using the following equilibrium: H$_2$PO$_4$$^-$ (aq) + H$_2$O $ ightleftharpoons$ H$_3$O$^+$ (aq) + HPO$_4$$^{2-}$ (aq); ($K_a$ = 6.2 x 10$^{-8}$) (b) Write an equation for the buffering reaction: (i) when a strong acid is added; (ii) when a strong base is added to the phosphate buffer solution. (c) Calculate the final pH of the solution: (i) when 2.0 mL of 6.0 M HCl(aq) is added to the buffer; (ii) when 2.0 mL of 6.0 M NaOH(aq) is added to the buffer. (In each case, assume the final volume of the solution is 250. mL)

          A phosphate buffer solution is prepared by dissolving 25.0 g of potassium hydrogen phosphate
(K$_2$HPO$_4$; FM = 174.18) and 12.0 g of potassium dihydrogen phosphate (KH$_2$PO$_4$; FM = 136.09) in
250. mL of the buffer solution.
(a) Calculate the molar concentrations of HPO$_4$$^{2-}$ and H$_2$PO$_4$$^-$, respectively, and the pH of the buffer
solution using the following equilibrium:

H$_2$PO$_4$$^-$ (aq) + H$_2$O $
ightleftharpoons$ H$_3$O$^+$ (aq) + HPO$_4$$^{2-}$ (aq);
($K_a$ = 6.2 x 10$^{-8}$)
(b) Write an equation for the buffering reaction: (i) when a strong acid is added; (ii) when a strong base
is added to the phosphate buffer solution.
(c) Calculate the final pH of the solution: (i) when 2.0 mL of 6.0 M HCl(aq) is added to the buffer; (ii)
when 2.0 mL of 6.0 M NaOH(aq) is added to the buffer. (In each case, assume the final volume of the
solution is 250. mL)

A phosphate buffer solution is prepared by dissolving 25.0 g of potassium hydrogen phosphate
(K2HPO4; FM = 174.18) and 12.0 g of potassium dihydrogen phosphate (KH2PO4; FM = 136.09) in
250. mL of the buffer solution.
(a) Calculate the molar concentrations of HPO4^2- and H2PO4^-, respectively, and the pH of the buffer
solution using the following equilibrium:

H2PO4^- (aq) + H2O ightleftharpoons H3O^+ (aq) + HPO4^2- (aq);
(Ka = 6.2 x 10^-8)
(b) Write an equation for the buffering reaction: (i) when a strong acid is added; (ii) when a strong base
is added to the phosphate buffer solution.
(c) Calculate the final pH of the solution: (i) when 2.0 mL of 6.0 M HCl(aq) is added to the buffer; (ii)
when 2.0 mL of 6.0 M NaOH(aq) is added to the buffer. (In each case, assume the final volume of the
solution is 250. mL)

Added by Kevin F.

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