Wednesday Quiz 3
Chem 265
2012
Name:
[1] 1.
The term masking generally refers to
(a) removing any material that may interfere with an analysis.
(b) the transformation of an interfering species into a form that is not detected.
(c) selecting samples from which the identity has been masked. This procedure
prevents the analyst from prejudging the outcome of the analysis.
[1]
2.
In a random heterogeneous material,
(a) differences in composition occur randomly and on a fine scale.
(b) large regions have obviously different compositions.
(c) samples are collected by taking portions from the desired number of
segments chosen at random.
[2]
3.
A 500.0 mL sample is concentrated for analysis to 50.0 mL. What is the correct
expression for the new concentration?
(a) [sample] x (50.0 mL/500.0 mL)
(b) [sample] x 100
(c) [sample] x (500.0mL/50.0 mL)
[5] 4.
A bulk lot containing 108 particles weighs 215 g.
All particles were the same size of 0.5 mm.
The mixture contained particles of cumin and cinnamon in a ratio of 1:90.
What is the expected number and relative standard deviation of cumin particles in a
sample of the above lot weighing 2.15 g?
Let n = expected number of particles in total sample.
n = 108 * 2.15/215 = 106
Fraction of cumin particles =
1
90+1
= 0.011 𝑜𝑟 0.01 = p
The number of cumin particles in the sample will be np = 1.1*104
or 104
𝜎 = √npq = √106 ∗ 0.011 ∗ 0.99 = √1.1 ∗ 104 = 1.0 ∗ 102
Relative standard deviation =
0.91% or 1 %
𝜎
𝑛𝑝
=
1.0∗102 ∗100%
1.1∗104
= 9.1 ∗ 10−1 % =