1
Chem 265
October
Midterm
2011
Name:
[1]
1.
What is the percentage of carbon dioxide in dry air?
[1]
2.
What is the [H+] in pure water in equilibrium with the air?
[4]
3.
Write equations for the dissolution of carbon dioxide in water.
[2]
4.
Why will a coral reef dissolve as the concentration of carbon dioxide in the air
above it increases?
2
[4]
5.(a) Calculate the energy of a photon of light with a wavelength of 325 nm.
h = 6.626 x 10-34 J.s
c = 2.997 x 108 m/s
[1]
(b) Is the 325 nm light in the visible region?
[1]
(c)
[3]
6.
How will the wavelength change as the light travels through seawater?
Write the reaction for the hydrolysis of ammonia in water.
Will the resulting solution be acidic or basic?
[1]
7.
Perchloric acid is often used to prepare samples for analysis.
Why is the use of perchloric acid considered hazardous?
[1]
Is perchloric acid used to decompose inorganic or organic samples?
3
[2]
[6]
7.(a) If you are going to sample a mix of iron ore and sand, what could you do to ensure
that the uncertainty in the sampling was as low as possible? Suggest 2 things.
(b) The ratio of the iron to sand particles is 1:4.
If we wish to analyze the iron ore, and want the sampling uncertainty for our iron
particles to be 2 %, how many particles should we sample?
[4]
[1]
(c) If the uncertainty in our analytical method for the iron ore is 4 %, what is the
overall uncertainty in the (sampling plus analysis)?
(d) The iron was dissolved and the silica needed to be removed (not just dissolved).
What could be done to achieve this?
4
[2]
[4]
(e)
(f)
The iron was analyzed by visible absorption spectrometry at 410 nm.
Phenanthroline was added to the solution of iron before the analysis. What was the
purpose of this?
If the molar absorptivity of the iron phenanthroline complex, Fe(II)
(phenanthroline)3, is 11,204 M-1.cm-1, the cuvette has a pathlength of 0.5 cm, and
the absorbance of the iron-phenanthroline solution analyzed was 0.534, what is the
concentration of the iron-phenanthroline complex –and thus the concentration of the
iron?
5
8.
You have a solution that is saturated with calcium fluoride and contains 0.0100 M
NaF.
[3]
Write the charge balance equation for this solution.
[3]
What concentration of calcium ion is present?
[2]
What would the calcium ion concentration be if no sodium fluoride was present?
6
[1]
9.
What is a gravimetric analysis?
[3]
10.
When I say that fluoride can be used to mask the interference of iron in an analysis
of cobalt using thiocyanate, what does this mean? How does the iron interfere?