Suppose that in a titration the following data are recorded when the solvent is

Question

Suppose that in a titration the following data are recorded when the solvent is pure water: [NaOH] = 0.080 M and 7.55 mL of NaOH are required to titrate a 50.00 mL aliquot of potassium hydrogen tartrate (KHTar). Calculate K_sp of the KHTar in this solvent. Sketch a titration curve for the titration of 100 ml of 0.0125 M acetic acid with 0.0100M NaOH. How many equivalence points the curve should have? Find out the pH at equivalence point? (pK_a = 4.75) Calculate the pH at 25 degree C of 1.00 L of a buffer that is 0.120 M HNO_2 and 0.150 M NaNO_2 before and after the addition of 1 mL of 12 M HCl. (pK_a = 3.39)

Explanation / Answer

When KHTar is dissolved in water, the following equilibrium exists

KHTar(s) « K +(aq) + HTar(aq)

The Ksp of KHTar in these solvents is given by:

Ksp = [K + ] [ HTar ]

In pure water the only sources of potassium and acid tartrate ions are the dissolved KHTar, which in view of the negligible amount of dissociation of HTar , leads to the following relation:

[K + ] = [HTar ]

Since HTar reacts with the sodium hydroxide in a 1:1 mole ratio:

HTar(aq) + OH (aq) ®Tar 2(aq) + H2O(l)

The moles of HTar are equal to the moles of NaOH used to reach the endpoint .

moles HTar = moles NaOH = (MNaOH) (Volume of NaOH solution)

= 0.080 Moles/L x 0.00755 L =0.000604 moles

[ HTar ] = no.of moles of NaOH / Volume of KHTar Solution

= 0.000604 moles / 0.05 L = 0.012 M

Ksp = [K + ] [ HTar ] = 0.012 x 0.012 = 1.45 x 10 -4

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