Slides exam 1

Question 1

What is the definition of analytical chemistry?

Answer 1

Analytical chemistry is the science of obtaining, processing, and communicating information about the composition and structure of matter.

Question 2

List the steps of chemical analysis

Answer 2

• Formulating the Question
• Selecting Analytical Procedures
• Sampling
• Sample Preparation
• Analysis of Sample
• Interpretation of Results
• Drawing Conclusions

Question 3

Define significant figures

Answer 3

The minimum number of digits required to express a value in scientific notation without loss of accuracy

Question 4

List an example of a) reproducible/systematic error and b) irreducible/random error

Answer 4

a) instrument calibration causes 0.01unit difference b) noise causes random difference in measurement

Question 5

What % of data in a gaussian distribution lies within 1, 2 and 3 standard deviations of the mean?

Answer 5

68.3, 95.5, 99.7

Question 6

What is the difference between linear range, dynamic range, and range?

Answer 6

Linear range: instrument response is proportional to concentration.

Dynamic range: instrument response is detectable (different) for each concentration, even if it’s not linear.

Range: entire spread of data.

Question 7

What is the matrix effect?

Answer 7

A change in analyte signal caused by anything other than analyte.

Question 8

What is Le Chatelier’s Principle?

Answer 8

Any change in a system not at equilibrium is such that it moves towards equilibrium.

Question 9

What is a Bronsted-Lowry acid-base pair?

Answer 9

An acid (contains an ionizable H) and its conjugate base (the anion when H is lost)

Question 10

What is a buffer? What is buffer capacity?

Answer 10

A buffer is a solution that resists changes in pH when an acid or a base is added to it. Buffers typically consist of a weak acid and its conjugate base (or a weak base and its conjugate acid) in solution.

A measure of how well the solution resists changes in pH when a strong acid or base is added

Question 11

(2) A student creates a calibration curve of the concentration of calcium in water using titration and FAAS. They measure 5 total samples then calculate a t value of 2.500. The students t-table value for a 95% confidence interval is 2.776. Do they reject or fail to reject the null hypothesis? Why?

Answer 11

Fail to reject – tcalc < ttable, so the null hypothesis (that the 2 sets of measurements are not different) can’t be rejected

Question 12

(2) Explain how to construct a representative sample from a) homogenous/random heterogenous material and b) segregated heterogenous material

Answer 12

a) divide into segments and collect random portions (called random sample)

b) divide into segments & collect a number of portions from each section in the ratio that they comprise the total sample (ex. 25% of samples come from the
material that makes up 25% of the total material) (called composite sample)

Question 13

(2) You collect measurements of 4.5, 9.2, 1.4, 5.6, and 8.0 and perform a q-test. Should 1.4 be discarded? Explain.

Answer 13

Q = 0.40. 0.40 < 0.64. Don’t discard 1.4.

If Q > Qtable, where Qtable is a reference value corresponding to the sample size and confidence level, then reject the questionable point.

Question 14

(2) Explain the difference and list a way to verify a) precision and b) accuracy

Answer 14

a) perform many repeats & look at st. dev.

b) compare to SRM

Question 15

(2) Lisa creates a calibration curve with [x] 2.0, 3.0, 4.5, 6.0 and 7.0 M. She measures her unknown and calculates [x] = 1.5 based on her calibration curve. Can she rely on this concentration?

Answer 15

No – she extrapolated beyond the curve. (She could solve by running another standard of 1.0 M to see if this value is within the linear range.)

Question 16

(2) A student creates a program to make calibration curves using the method of least squares. The program minimizes the sum of the square of the difference in x values. Is this approach correct? Why or why not?

Answer 16

This is incorrect – x-values represent standards, so x-errors are much smaller than y-errors. The method of least squares aims to minimize the sum of the square of y differences.

Question 17

(2) Based on the quality control chart, if 0.5% of your data is outside the action lines, and 6.7% is outside the warning lines, what should you do? Why?

Answer 17

They should use a new method. Only 0.3% of data should be outside the action lines, and 4.5% of data should be outside the warning lines.

Question 18

(2) You measure the content of riboflavin in beer using HPLC but cannot distinguish the riboflavin peak. How can you solve this issue to create a calibration curve?

Answer 18

Add riboflavin standard to your sample in increments and measure the response.

Question 19

(2) Why could a sample below the detection limit give a negative instrument response?

Answer 19

Below the detection limit your sample is not significantly different from the blank so you’re just measuring noise.

Question 20

(2) What is suggested by a large K value and why?

Answer 20

A large K means the reaction heavily favors the products at equilibrium because K = [P]/[R].

Question 21

(2) Why do you need to know the Keq to find the concentration of a weak acid’s constituents, but not a strong acid?

Answer 21

Strong acids dissociate completely, so [A] and [H] are equal to initial [HA].

Question 22

(2) What is the predominant form of acetic acid (pKa = 4.76) at pH = 5?

Answer 22

Acetate anion – pH > pKa, meaning there are few enough H+ ions in solution that the acid will dissociate

Question 23

(3, 1) What is the correct answer with the correct number of sig figs: (1.290 x 10E3) + 70,001 – (9.742 x 10E2)

Answer 23

70,317

Question 24

(3, 3) Analysis of your unknown tells you it contains 5.00 µM quercetin. You spike it with 15.0 µM quercetin, and your instrument reads a signal corresponding to 18.6 µM quercetin. What is the % spike recovery?

Answer 24

100*[(18.6-5)/15] = 90.7%