Slides exam 1 Flashcards
What is the definition of analytical chemistry?
Analytical chemistry is the science of obtaining, processing, and communicating information about the composition and structure of matter.
List the steps of chemical analysis
- Formulating the Question
- Selecting Analytical Procedures
- Sampling
- Sample Preparation
- Analysis of Sample
- Interpretation of Results
- Drawing Conclusions
Define significant figures
The minimum number of digits required to express a value in scientific notation without loss of accuracy
List an example of a) reproducible/systematic error and b) irreducible/random error
a) instrument calibration causes 0.01unit difference b) noise causes random difference in measurement
What % of data in a gaussian distribution lies within 1, 2 and 3 standard deviations of the mean?
68.3, 95.5, 99.7
What is the difference between linear range, dynamic range, and range?
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.
What is the matrix effect?
A change in analyte signal caused by anything other than analyte.
What is Le Chatelier’s Principle?
Any change in a system not at equilibrium is such that it moves towards equilibrium.
What is a Bronsted-Lowry acid-base pair?
An acid (contains an ionizable H) and its conjugate base (the anion when H is lost)
What is a buffer? What is buffer capacity?
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
(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?
Fail to reject – tcalc < ttable, so the null hypothesis (that the 2 sets of measurements are not different) can’t be rejected
(2) Explain how to construct a representative sample from a) homogenous/random heterogenous material and b) segregated heterogenous material
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)
(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.
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.
(2) Explain the difference and list a way to verify a) precision and b) accuracy
a) perform many repeats & look at st. dev.
b) compare to SRM
(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?
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.)
(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?
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.
(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?
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.
(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?
Add riboflavin standard to your sample in increments and measure the response.
(2) Why could a sample below the detection limit give a negative instrument response?
Below the detection limit your sample is not significantly different from the blank so you’re just measuring noise.
(2) What is suggested by a large K value and why?
A large K means the reaction heavily favors the products at equilibrium because K = [P]/[R].
(2) Why do you need to know the Keq to find the concentration of a weak acid’s constituents, but not a strong acid?
Strong acids dissociate completely, so [A] and [H] are equal to initial [HA].
(2) What is the predominant form of acetic acid (pKa = 4.76) at pH = 5?
Acetate anion – pH > pKa, meaning there are few enough H+ ions in solution that the acid will dissociate
(3, 1) What is the correct answer with the correct number of sig figs: (1.290 x 10E3) + 70,001 – (9.742 x 10E2)
70,317
(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?
100*[(18.6-5)/15] = 90.7%