Chapter 4
Critical Thinking Exercise
Solution Chemistry and the Hydrosphere
The Kjeldahl analysis is used to determine the protein content of foods. The food analyzed is
heated with sulfuric acid in the presence of a catalyst. The carbon in protein is converted to CO2,
the hydrogen to water, and the nitrogen to (NH 4)2SO4. The mixture is then treated with
concentrated NaOH and heated to drive off the NH3 gas. The NH3 gas is neutralized with excess
standard acid, and the acid present after the neutralization is titrated with standard base. From the
titration data and the known mass of the sample, the %N in the food sample can be calculated.
The average %N in most plant and animal protein is 16%.
A 2.500-g sample of meat is subjected to Kjeldahl analysis. The liberated NH3 gas is absorbed
in 50.00 mL of H2SO4 (aq). The excess acid requires 19.90 mL of 0.5510 M NaOH for its
complete neutralization. A separate 25.00-mL sample of the H2SO4 (aq) requires 22.65 mL of
the 0.5510 M NaOH for its titration.
1.
To determine the molarity of the H2SO4, what conversion factor is used? (Write the
balanced chemical reaction equation between sulfuric acid and sodium hydroxide.)
a.
mol H2 SO 4
2 mol NaOH
b.
2 mol H2 SO 4
mol NaOH
c.
2. What is the molarity of the sulfuric acid?
a. 0.1238 M
b. 0.2496 M c. 0.3499 M
2 mol NaOH
mol H2 SO 4
d. 0.4357 M
d.
mol NaOH
2 mol H2 SO 4
e. 0.4992 M
3. What is the total moles of acid in 50.00 mL of H 2SO4 solution?
a. 0.00619 mol
b. 0.02179 mol
c. 0.01248 mol
d. 0.01750 mol
e. 0.02496 mol
4. How many moles of excess acid are present after the neutralization by ammonia?
a. 0.005482 mol
b. 0.002214 mol
c. 0.003489 mol
d. 0.007651 mol
e. 0.006532 mol
Chapter 4 – Reactions in Aqueous Solutions
5. How many moles of acid were neutralized by the ammonia?
a. 0.01027 mol
b. 0.01843 mol
c. 0.00483 mol
d. 0.00899 mol
e. 0.00700 mol
6. How many moles of NH3 did the amount of acid in Question 5 neutralize? (Write the
balanced chemical reaction equation between sulfuric acid and ammonia.)
a. 0.01843 mol
b. 0.00899 mol
c. 0.02054 mol
d. 0.01400 mol
e. 0.00966 mol
7. How many moles of N are present for the given number of moles of ammonia in
Question 6?
a. same number
b. twice the number
d. three times the number
c. half the number
e. one third the number
8. How many grams of N are present in the meat?
a. 0.2582 g
b. 0.1961 g
c. 0.1353 g
d. 0.2878 g
e. 0.1259 g
9. What is the percent N in the meat?
a. 11.51 %
b. 5.036 %
c. 7.844 %
d. 5.412 %
e. 10.33 %
10. What is the percent protein in the meat?
a. 71.94%
b. 64.56 %
c. 49.03 %
d. 33.83 %
e. 31.38 %