Statistical analysis assignment ( 5 marks eah)
1. Calculate the uncertainty in the number of millimoles of chloride contained in 250.0 mL
of a sample when three equal aliquots of 25.00 mL are titrated with silver nitrate with the
following results: 36.78, 36.82, and 36.75 mL. The molarity of the AgNO3 solution is
0.1167 ± 0.0002M.
2. Using tolerance values from Table below. Which of the following methods for preparing a
0.0010 M solution from a 1.0 M stock solution provides the smallest overall uncertainty?
(a) A one-step dilution that uses a 1-mL pipet and a 1000-mL volumetric flask.
(b) A two-step dilution that uses a 20-mL pipet and a 1000-mL volumetric flask for
the first dilution, and a 25-mL pipet and a 500-mL volumetric flask for the second
dilution.
3. Analysis of several plant-food preparations for potassium ion yielded the following data:
Sample Percent K+
1
6.02, 6.04, 5.88, 6.06, 5.82
2
7.48, 7.47, 7.29
3
3.90, 3.96, 4.16, 3.96
4
4.48, 4.65, 4.68, 4.42
5
5.29, 5.13, 5.14, 5.28, 5.20
The preparations were randomly drawn from the same population.
(a) Find the mean and standard deviation s for each sample.
(b) Obtain the pooled value spooled.
(c) Why is spooled a better estimate of s than the standard deviation from any one
sample?