=========================preview======================
(CHEM141)[2010](f)midterm~id-^_10033.pdf
Back to CHEM141 Login to download
======================================================
CHEM 141 Analytical Chemistry I
Mid-Term Examination
Wednesday 13 October 2010 Time allowed 2 hrs Total 100pts

Answer all questions on the answer grid provided. You may carry out rough working in the exam booklet but this will not be collected and graded.
Section 1. Multiple Choice (28pts)
1.1 Units (4pts)
Which two of the following are SI units named after people ?
a) ohm b) candela c) Mandela
d) Calorie e) litre f) joule
a) ohm and f) joule

(4pts for both, 3 pts for just 1, 2pts for 1 plus 1 incorrect)

1.2 pH and pKa (4pts)
Which two statements about pH or pKa are false ?
a) The pH of 1.0 M hydrochloric acid (HCl) solution is approximately 1.0.
b) For a mono-protic weak acid [HA], when solution pH = pKa the concentrations

of [HA] and its conjugate base [A-] are equal. c) A pH of 12.327 has 3 significant figures. d) The pKa of water is approximately 14.0. e) If the pKa of acetic acid is 4.756 then the pKb of acetate is 9.244. f) None are false.
a) 1M HCl pH should be 0.0 and d) pKa water 15.74 (4pts for both, 3 pts for just 1, 2pts for 1 plus 1 incorrect)

1.3 Solubility product (4pts)
The solubility product Ksp for lead iodide PbI2 is 7.9 x 10-9. What is the concentration (mM) of iodide ions in a saturated solution of the compound under standard conditions ?
a) 1.25 b) 2.51 c) 2.63 d) 3.98 e) none are correct f) cannot be calculated from this information.
Ksp = [Pb][I][I] if x = [Pb] then [I] = 2x Ksp = 4x3 x = (7.9E-0/4)1/3
x = 1.25 E-3 this is [Pb] 2x = 2.51E-3 this is [I] Answer b) 4pts answer a) 2pts

1.4 Accuracy and Precision (4pts)
In analytical chemistry, which two statements are false ?
a) A more precise answer is one that is closer to the true value.
b) An accurate answer can only be obtained after using several different methods.
c) An answer that is reproducible using the same method is termed precise.
d) Systematic error affects accuracy, but can be often be corrected for.
e) Random error affects precision and can be reduced, but not corrected for.
f) None are false.

Answer a) should state more 'accurate' and b) several methods may be used to uncover possible systematic error, but this may not be necessary (4pts for both, 3 pts for just 1, 2pts for 1 plus 1 incorrect)

1.5 Concentration Types (4pts)
Which statement a-d) about Molar (M), molal (m), Normal (N) and Formal (F) type concentrations is false?
a) 0.1 M H2SO4 is equivalent to 0.2 N H2SO4 b) 0.1 M H2SO4 is not equivalent to 0.1 m H2SO4 c) 0.1 M H2SO4 should be more appropriately described as 0.1 F H2SO4 d) The molarity of a solution does not change with temperature if evaporation is
prevented. e) All statements a-d) are correct f) More than one statement a-d) false
Statement d) since volume will expand with T, so molar conc will decrease
Statement c) or e) 1pt

1.6 Gravimetric Methods (4pts)
Cyanide ion CN -can be measured through gravimetry by addition of