CHE 322 Spring 2017 Homework 4 Full Name (please print) Due date: 4/7/2017 Date
ID: 509723 • Letter: C
Question
CHE 322 Spring 2017 Homework 4 Full Name (please print) Due date: 4/7/2017 Date submitted 1) 25.00 mL of a 0.0060 M solution of sodium hydroxide was titrated with 0.010 M hydrochloric acid using methyl red indicator. a. Calculate the volume ofHCl at equivalence point. b. Draw a rough sketch of the titration curve (pH vs c. Calculate ApH and show the transition pH of the indicator on the pH-curve. d. Calculate Vrand show vrand Veg on the pl curve. Calculate the indicator emor. Methyl red: Red 46-62 YellowExplanation / Answer
1) Titration of string acid versus strong base
NaOH + HCl --> NaCl + H2O
a) moles of NaOH present = 0.006 M x 25 ml
= 0.15 mmol
moles of HCl required = 0.15 mol
Volume of HCl needed to reach equivalence point = 0.15 mol/0.01 M
= 15 ml
b) Titration plot,
equivalence point at pH 7
2) moles of NaOH used = 0.0998 M x 20.25 ml
= 2.02095 mmol
1 mole of ascorbic acid uses 2 moles of NaOH for neutralization
therefore,
moles of asocrbic acid present = 2.02095/2
= 1.010475 mmol
mass of ascorbic acid present = 1.010475 mmol x 176.124 g/mol = 0.178 g
mass% of ascorbic acid in sample = 0.178 g x 100/1.0567 g = 16.845%
3) moles of NaOH to reach eqivalence point = moles of acid present
= 0.02465 M x 0.03765 L
= 0.00093 mol
when,
NaOH = 0.02465 M x 0.01087 L = 0.000268 mol was added,
pH = 5.87
moles of acid remained = 0.00093 - 0.000268 = 0.000662 mol
pH = pKa + log(base/acid)
5.87 = pKa + log(0.000268/0.000662)
pKa = 6.263
So the pKa of acid = 6.263