AP Chem worksheet: Solutions!
Page 1
Colligative properties: (for non-volatile solutes)
Vapor Pressure : of a solution is lower than the pure solvent!
Boiling point elevation: ! ∆Tb = Kb m !
Freezing point depression: ∆Tf = Kf m
(***See Kb and Kf chart on the Solution Handout***)
1. Calculate the freezing point of a 3.00 m solution of sugar in water. !
2. a. What is the molality of 0.300 mol of asprin in 250 g of chloroform? !
!
b. What are the freezing and boiling points this solution?!
(–5.58°C)
(1.20 m)
(–69.1°C, 65.6°C)
3. Calculate the grams of C2H6O2 (anti freeze, a non–electrolyte) that must be added to 120 g of
water (the solvent) to reduce the freezing point to – 40°C. ( which is also –40° F)!
(160 g)
Osmotic Pressure of a solution : $ = MRT
4. What is the osmotic pressure in mmHg at 20°C of a 0.0020 M solution of glucose, C6H12O6
!
(0.0481 atm, 36.5 mmHg)
5. The osmotic pressure of blood is 7.7 atm at 25°C. What concentration of glucose, C6H12O6, is
isotonic (same osmotic pressure) with blood?!
(0.315 M)
6. Urea, (NH2)2CO, is the product of protein metabolism in mammals. The osmotic pressure of
100 mL of a urea solution in water is 4.4 atm at 20.0 °C. How many grams of urea are in the
100 mL of solution?!
(1.10 g)
7. a. Describe what happens when a cell is put into a hypotonic solution.
!
b. Describe what happens when a cell is put into a hypertonic solution.
Strong electrolytes !
Page 2
8. Write the equation for these substances dissolving in water. Then calculate the molality of the
particles in these solutions. (Some are strong electrolytes, some are not).
!
a. 0.100 m KCl!
(0.200 m)
!
b. 0.100 m C12H22O11 !
(0.100 m)
!
c. 0.100 m K2SO4!
(0.300 m)
!
d. Which of the above solutions would have the lowest freezing point? The highest? (Don’t
calculate, just think!)
9. a. What is the molality of 75 g of NaCl in 600 g H2O? !
(2.14 m)
!
b. What is the molality of the particles?!
(4.28 m)
!
c. What are the freezing and boiling points of this solution?!
!
!
(–7.96°C
102.23°C)
10. How many grams of CaCl2 (a strong electrolyte) do you need to add to 350 g of water so the
freezing point of the mixture will be –9.50°C (about 15°F). !
(66 g)
11. What is the osmotic pressure of seawater at 25°C? Assume that seawater contains 34 g of
NaCl per liter of solution.!
(28.4 atm)
b. Why shouldn’t you drink seawater? (See Q. 5 & 7 on front)
7a KCl(s) ––> K+ + Cl– ; b. C12 H22O11(s) ––> C12 H22O11(aq) ; c. K2SO4(s) ––> 2 K+ + SO4–2 , d. c lowest, b highest)
AP Chem worksheet: Solutions!
Page 3
Determining molecular weights
1. a. 69.0 g of an unknown non-electrolyte dissolved in 500 g of water has a freezing point of
!
–2.79°C. What is the molecular weight of the unknown?!
(92 g/mol)
!
b. If the empirical formula of this compound is C2H6O, what is its molecular formula?
2. 25.0 g of an unknown non-volatile, non-electrolyte is dissolved in 130 g of water. The boiling
point of the solution is 102.5 °C. What is the molecular weight of the unknown?! (40 g/mol)
3. a. When 0.175 g of an organic (non-electrolyte) substance is dissolved in 22.0 g of liquid
camphor, the freezing point of the mixture is found to be 176.7 °C. What is the molecular
weight of the solute? (See Kf chart)!
(96 g/mol)
!
b. If the empirical formula of this compound is CHF, what is its molecular formula?
4. The osmotic pressure of of an aqueous solution of a protein was found to be 1.54 mm Hg at
25°C. The solution contains 3.50 mg of protein in 5.00 mL of solution. Calculate the molecular
weight of the protein. !
(8.4 x103 g/mol)
5. Calculate the grams of C2H6O2 (anti freeze, a non–electrolyte) that must be added to 120 g of
water to reduce the freezing point to – 40°C. ( which is also –40° F)!
(160 g)
Lab Example Question!
Page 4
!
You want to identify an unknown substance, so you decide to use the freezing point
depression of benzene. First you measure the Freezing point of pure benzene. Then you add
1.93 g of the solute to 20.0 g of benzene. The graph of your data is shown below.
1. What’s the freezing point of the pure benzene? __________ The mixture?_________
2. Calculate the freezing point depression. _______________
3. Using the freezing point depression, calculate the molality (m) in mol/kg of the unknown
solute in the mixture. Kf for benzene is 5.12°C/m.
!
∆Tf = Kf m
4. Using your mass of solute and benzene and the above calculated molality, calculate the
experimental molecular weight of your unknown in g/mol.
5. Based on this experimental (approximate) molecular weight, which of the listed substances are
possibilities for your substance?
6. 250 mg of your substance is analyzed. It contains 123 mg C, 24.0 mg H, 48.0 mg N and 54.7 mg
O. Determine the empirical formula for the compound.
7. Based on your empirical formula and your molecular weight, what is the molecular formula for
the compound? Which substance is it?
Aspartame = Nutrasweet
C14H18N2O5, 294 g/mol
Aspartic acid, =an amino acid
C4H7NO4, 133 g/mol
Lysine = an amino acid
C6H14N2O2, 146 g/mol
Para Amino Benzoic acid,
(PABA, a former ingredient in
sunscreens),
C7H7NO2, 137 g/mol
L-Tryptophan = an amino acid
C11H13N2O2, 205 g/mol
Tetracycline = an antibiotic
C22H24N2O8, 444 g/mol
Chromatography
After a chromatography experiment, a student develops this chromatogram.
Point of
Solvent
application
Front
1. For the above chromatogram: !What is the value for L: cm solvent moved ____________
!
What are the values for D: cm ion moved 1st ion_______________ 2nd ion ______________
2. Calculate the Rf value for each ion in the mixture. Rf = D/L
3. Mn+2 has an Rf value of 0.50 under the same conditions as the above experiment. Was Mn+2
present in the solution that produced this chromatogram? Explain.
4. A sample moves 3 cm in about 5 minutes. Why shouldn’t the experiment be stopped at at that
time instead of waiting 20 minutes for the solvent to move 10 cm?
5. You have a polar solute. Will it move far up the paper or not far up the paper? Explain.
6. Classify these solvents as Polar or Non-polar:
Benzene (C6H6)!
Water!
Pently alcohol (C5H11OH)!
Ethyl alcohol, C2H5OH
Liquid Ammonia (NH3)
Spectroscopy
1. Here is the Absorbance vs.
Wavelength graph for Blue 1 dye.
What is the best wavelength to use
for analysis of this dye? Why?
2. What is the relationship between Absorbance and % Transmittance?
Here is a Beer’s Law graph for Blue 1 dye.
3. A sample has absorbance of 0.40 . What is
its approximate concentration? _________
b. If you had a sample with absorbance 1.4,
could you determine its concentration?
Explain.
4. You have a 100 µM stock standard solution. You need to make a 6.5 µM standard solution.
Briefly describe how you would do this. Use correct terms for glassware.
5. You make the 6.5 µM solution and determine its absorbance is 0.35. Add this point to the
graph. Comment on your Data. Does it seem OK? What might you have done worng?