Ka and pH Problems
1. Possible Calculations
2. Sample Questions
i).Write the ka expression for each of the following:
HBr , HNO2 (nitrous acid) , HPO42- (monohydrogen phosphate)
ii)Write the ka expressions for the following polyprotic acids
H2SO4 , H3PO4
iii)A) Formic acid (HCHO2), is a monoprotic acid. In a 0.100 M solution of formic acid, the pH is 2.38 at 25oC. Calculate the ka for this temperature.
B) When butter turns rancid, its foul odour is mostly that of butyric acid, a weak acid. A 0.0100 M solution of butyric acid has a pH of 3.40 at 20oC. Calculate the ka for this temperature.
C) At 60oC, the pH of 0.0100 M butyric acid is 2.98 . Calculate the ka at this temperature.
iv)Simplifications for weak acids
A) The concentration of a sample of vinegar was found to be 0.75 M acetic acid. Calculate the values of [H+] and pH of this sample. ka=1.8x10-5
B) Nicotinic acid, HC2H4NO2, is a B vitamin. it is also a weak acid with ka=1.4x10-5. What is the [H+] and the pH of a 0.010 M solution?
v)Percent Ionization:
A) Calculate the % ionization in 0.10 M acetic acid.
B) Calculate the % ionization of 0.010 M and 0 0010 M acetic acids.
ka and pH Problems
1. 6.1 g of benzoic acid, C6H5COOH, is dissolved in 5.0L of distilled water. The pH of the solution is 5.3. Find the ka. (2.5x10-9)
2. An acid , HA has a ka=4.6x10-2. If a solution of HA has a pH of 3.3, find the concentration of the HA solution.
A) Using the assumption that very little HA ionizes.(5.4x10-6)
B) Since ka is actually fairly large, this assumption is not valid. Calculate the [HA] without the simplification. (5.0x10-4)
3. What is the pH of a 0.010 M NH3 solution.
Assume only 4.0% ionizes.(pH=10.6)
4. What is the pH of a 0.0010 M solution of carbonic acid? (diprotic)
Note:
Hint: One of these reactions may be ignored.(pH=4.7)
5. What is the pH of a 0.0010 M solution of phosphoric acid ?
ka values are 1.1x10-2 , 7.4x10-8 , 4.8x10-13(pH=3)
6. Acid HA has a molar mass of 120 g/mol and ka=1.0x10-4.
Acid HB has a molar mass of 190 g/mol and ka= 1.0x10-6.
A 1.00 g sample is made up to 50.0 mL of solution. Find the pH of this solution given that it takes 16.7 mL of 0.50 M NaOH to neutralize the 50.0 mL of the acid. (pH=2.4)
Concentration Problems
1. A) Write the balanced equation for the neutralization of sodium hydroxide with sulphuric acid.
B) How many moles of NaOH is required to neutralize 0.40 moles of H2SO4 (0.80)
C) What volume of 0.50 M NaOH will neutralize 30 mL of 0.15 M H2SO4?(56.3g)
2. A) Calculate the molarity of a solution containing 100 g of hydrogen sulphate (H2SO4 ) in 250 mL of solution. (4.08M)
B) Calculate the molarity of a solution containing 2.8 g of potassium hydroxide in 250 mL of solution. (0.20M)
C) What volume of the solution referred to in "A" will be required to neutralize
45 mL of the solution referred to in "B" ?(1.1mL)
3. 20.0 mL of 3.0 M NaOH solution neutralized 300 mL of phosphoric acid solution.
A) Calculate the molarity of the H3PO4 solution.(0.067M)
B) The [H3PO4 ] in g/L .(6.57g/L)
4. What volume of 0.10 M NaOH solution would be required to neutralize 0.196 g of hydrogen sulphate ? (0.040L)
5. What is the concentration of acid obtained by diluting 20 mL of 3.5 M acid to 250 mL ? (0.28M)
6. A solution of NaOH is 20% by mass. The density of the solution is 1.2 g/cm3. Calculate the molarity. (6.0M)
7. A bottle of concentrated nitric acid is labelled: 70% by mass , specific gravity =1.42.
A) What is the [HNO3] ?(15.8M)
B) What volume of this concentrated acid is required to make , 5.0 L of 1.5 M solution ? (0.47L)
8. 20.0 mL of stock HCl ( S.G.=1.18 , 35% by mass ) is made up to 0.500 L by adding water. What volume of this acid will be required to neutralize 60.0 mL of 0.50 M KOH?(66.7mL)
9. What volume of 0.50 M NaOH is needed to neutralize a 1.20 g sample of a monoprotic acid with a molar mass of 240 g/mol ? (0.010L)
10. A sample of monoprotic solid acid with a mass of 0.99 g is neutralized by 14.6mL of 0.88 M NaOH solution. Calculate the molar mass of the solid acid. (77g/mol)
11. A 0.450 g sample of impure CaCO3 was reacted with 50 mL of 0.0985 M HCl. The excess acid was neutralized with 6.00 mL of 0.105 M NaOH. Calculate the % CaCO3 in the sample.