IN-CLASS PROBLEMS – Solutions
Percent By Mass:
Ex. 1: [Calculating % (w/w)] Biological specimens are preserved in solutions prepared by dissolving 667 g of formaldehyde in 1000. g of water. What is the concentration of the solution, in % (w/w)?
40.0 % (w/w)
Ex. 2: [Using % (w/w)] Describe the procedure for making up 25.0 grams of a 5.00 % (w/w) KOH solution in ethanol. Hint: How much solute and solvent will you need?
Dissolve 1.25 g KOH in 23.8 g of ethanol
Ex 3: What is the % (w/w) for a solution containing 10.55 grams of AgNO3 dissolved in 500. mL of water?
2.07% (w/w) AgNO3
Ex 4: If you add 5.0 g of NaCl to a 250.0 mL volumetric flask, and fill it to the mark with water, what is the molar concentration of the resulting solution?
Ex 5: What is the molarity of a solution containing 15 grams of NaOH in 100. mL solution?
Calculations Using Molarity:
Ex 6: How many moles of NaCl are needed to prepare 500.0 mL of a 2.5 M solution?
1.3 moles NaCl
Ex 7: How many milliliters of a 0.15 M NaCl solution contain 0.03 moles of NaCl?
2 x 102 mL sol’n
Ex 8: You have a 55 mL sample of a 0.100 M KOH solution. How many grams of solute are present?
0.31 g KOH
Ex 9: How many pL of a 0.45 M BaCl2 solution contain 1.50x103 centigrams of solute?
1.6 x 1011 pL sol’n
Ex. 10: How many moles of chloride ions are present in 55 mL of a 0.567 M AlCl3 solution?
0.094 moles Cl-
Stoichiometry involving Solutions:
2 NaOH + H2SO4 à Na2SO4 + 2H2O 0.598 grams of NaOH
Ex. 12: Consider the unbalanced reaction: Pb(NO3)2 (aq) + K2CrO4 (aq) à KNO3 (aq) + PbCrO4 (s)
a) How many grams of PbCrO4 are produced from 154 mL of 0.180 M Pb(NO3)2, according to the reaction above?
Pb(NO3)2 (aq) + K2CrO4 (aq) à 2 KNO3 (aq) + PbCrO4 (s) 8.96 grams of PbCrO4
b) How many grams of KNO3 can also be produced?
5.61 grams of KNO3
Ex. 13: Some cleaners contain HCl. A 0.010 L sample of cleaner reacts completely with 242 mL of a 0.248 M NaOH solution. What is the molarity of the HCl in the cleaner?
0.0600 mol HCl / 0.010 L HCl solution = 6.0 M HCl
Ex. 14: Consider the unbalanced reaction: Pb(NO3)2 (aq) + KCl (aq) à KNO3 (aq) + PbCl2 (s)
a) How many liters of a 0.430 M solution of potassium chloride solution would be needed to react with 154 mL of 0.180 M Pb(NO3)2?
0.129 L KCl soln
b) If 250. mL of 0.450 M Pb(NO3)2 was reacted with 525. mL of 0.450 M KCl, how many moles of precipitate would result?
Ex 15: How many mL of a 6.5 M H2SO4 solution are needed to prepare 600.0 mL of a 3.6 M solution?
330 mL of 6.5 M sol’n
9.69 % (w/w)
Ex 17: Describe how you would prepare 2.0 L of 6.0 M HNO3 from concentrated (16 M) nitric acid.
Dilute 0.75 L of 16 M HNO3 to the 2.0 L mark of a volumetric flask