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

 

Calculating Molarity:

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? 

 

0.34 M

 

 

Ex 5: What is the molarity of a solution containing 15 grams of NaOH in 100. mL solution? 

 

3.8 M

 

 

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:

 Ex 11: A flask contains an unknown amount of dissolved NaOH.  If you completely neutralizes the solution using 48.6 mL of a 0.154 M H2SO4 solution, how many grams of NaOH were present?  (The MM of NaOH and H2SO4 are 40.01 and 98.06 g/mol)

 

  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? 

0.113 moles

 

 

Dilutions:

 

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

 

 Ex 16:  What is the final concentration of a solution obtained by mixing 150. mL of water with 250. mL of a 15.5% (w/w) ethanol solution? 

 

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