Exam I Practice exam * please complete by 5/5/16 9:00pm 1.To prepare a solution

Question

Exam I Practice exam * please complete by 5/5/16 9:00pm

1.To prepare a solution of NaCl (FW 58.443 ± 0.001) a total of 1.265 (± 0.002) g is weighed out and diluted in a 25.00 (± 0.03) mL volumetric flask. What is the molarity (± uncertainty) of the solution?

(25 pts.) Name:_____________________ TA:_________________________

2. Sulfamic acid (FW 97.094) is a primary standard that can be used to standardize NaOH.

+H3NSO3- + OH- H2NSO3- + H2O

a. How much primary standard was weighed out for the standardization titration if 24.65 mL of 0.9937 M NaOH was used to reach the endpoint in the standardization titration? (30 pts.)

b. If three replicates of the blank titration gave a 4.32 (±0.01) mL and the standardization titration was repeated two more times for an average volume of 24.67 (±0.03) mL. What is the equivalence volume? (20 pts.)

Name:_____________________ TA:_________________________

c. An entire Chem 222 section (all 21 students) used the same NaOH solution (labeled 0.9937 M) and found an average concentration of 0.9942 (±0.0005) M NaOH for. Is this significantly different at the 95% confidence level from stockroom label? (25 pts.)

Name:_____________________ TA:_________________________

3. a. The method of standard additions with a constant volume was used to in an experiment to measure the amount of nitrate in water from a fish tank. To each sample of tank water 0, 1.0, 2.0, 3.0 and 4.0 mL of 3.0 ppm nitrate were added. The data for each sample collected and plotted against the volume of added nitrate standard. The output from the Excel LINEST command is shown below. If each sample of tank water was 10.0 mL, what was the concentration of the nitrate in the tank water? (20 pts.)

Slope Intercept Parameter 2.939867 7.030828 Std Dev 0.229437 0.562003 R^2 0.982056 0.725543 Std Dev (y)

b. A colorimetric method was explored for the possibility of determining nitrate in the fish tank water. A series of nitrate-free samples were analyzed to give a signal of 0.106 units. An additional series of low concentration nitrate standards gave signals of 0.203, 0.200, 0.198, 0.206, 0.202, 0.198, 0.199 units. If the linear calibration regression is y (units) = 0.017 x (units/ppm) + 0.058 (units), will this method be able to be used to accurately quantitate nitrate in the aquarium water? (Show why or why not.) (25 pts.)

4. Chromate is being used to separate 0.010 M copper and 0.350 M silver ions from a solution. Is it possible to precipitate 99.999% of the ion that precipitates first? (35 pts.)

b. Under standard conditions what is the Gibbs Free Energy of the precipitation reaction for the ion that precipitated? (20 pts.)

Explanation / Answer

1.

First, let's calculate the moles:
moles = 1.265/58.443 = 0.0216 moles
uncertainty: = 0.002/1.265 + 0.001/58.443 = 0.0008

Now the molarity:
M = 0.0216/0.025 = 0.864 M
uncerainty = 0.0008/0.0216 + 0.03x10-3/0.025 = 0.0382 or simply 0.04 M

2. a) calculate the moles:
moles =0.9937 * 0.02465 = 0.0245 moles

These moles are the same of the sulfamic acid at equivalence point, so the mass:
m = 0.0245 * 97.094 = 2.3788 g

b) If 3 replicatesof the blank gave 4.32 mL and the titration gave 24.67 mL, all you need to do is rest those values:
V = 24.67 - 4.32 = 20.35 mL
the uncertainty: 0.03+0.01 = 0.04 mL

The other two questions post them in another question thread.

Hope this helps

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