Question

question 1.Your TA assigns you two monoprotic( one proton per molecule) acids HA and HB. You are given 43.5 mL of HA solution in the first flask. A second flask contains 37.2 mL of HA and enough HB solution is added to reach a final volume of 50.0mL. You titrate the HA solution, in the first flask, with 87.3mL of 0.0906 M NaOH. What is the molarity of the acid HA? ( Write answer to two decimal places, ex. 1.25 )

question 2.Your TA assigns you two monoprotic( one proton per molecule) acids HA and HB. You are given 43.5 mL of HA solution in the first flask. A second flask contains 37.2 mL of HA and enough HB solution is added to reach a final volume of 50.0mL. You titrate the second flask, containing HA and HB with 96.4mL of 0.0906 M NaOH. What is the molarity of the acid HB?  ( Write answer to two decimal places, ex. 1.25 )

( Hint: You will need to use the answer from the previous question)

Answer 1

Given : Two monoprotic acids :

Volume in first flask : HA = 43.5 mL = 0.0435 L

Volume in second flask : HA : 0.0372 L + x mL HB = 50.0 mL

Volume of NaOH = 0.0906 M volume of NaOH = 87.3 mL = 0.0873 L

Molarity of acid A

Lets write the reaction between HA and NaOH

NaOH (aq) + HA (aq) --- > NaA (aq) + H2O (l)

Molarity of acid HA = Number of moles HA / volume in L

Number of moles of NaOH = Molarity x volume in L

= 0.0906 M x 0.0873 L = 0.007909 mol

Moles of acid = 0.007909 mol NaOH x 1 mol HA / 1 mol NaOH

= 0.007909 mol HA

[HA]= 0.007909 mol / 0.0435 L = 0.182M

Q. 2 :
50.0 mL = 43.5 mL HA + x mL HB

Volume of NaOH needed for the titration of second flask = 96.4 mL

Molarity of NaOH = 0.0906 M

Moles of NaOH = 0.0906 M x 0.0964

= 0.0087 mol NaOH

Lets calculate moles of acid from flask two

Moles of acid = moles of NaOH x 1 mol acid (HA + HB)/ 1 mol NaOH

= 0.0087 mol (HA+ HB)

Number of moles of (HA+ HB) = 0.0087

Lets calculate moles of HA

Moles of HA = molarity x volume in L = 0.182 M x 0.037 L = 0.006734 mol

Moles of HB = 0.0087 – 0.006734 = 0.002 mol

[HB]= 0.002 mol / (0.050-0.0370) = 0.154 M