1 Multistep practice problems answer key 1. A technician has prepared a growth medium for bacteria that requires 3500 mg of lactose per 500 mL of medium. The technician added 6 grams of lactose to make 7 liters of medium. Did the technician make this medium correctly? Technician should have added: 3500 mg lactose / 500 mL medium = 7 mg lactose / mL medium For 7 liters (7000 mL), technician should have added: 7 mg lactose = X mL medium 7000 mL medium X = 7 mg lactose(7000 mL) = 49,000 mg lac. or 49 g of lactose mL medium Technician added 6 grams per 7 liters which is way too little. 2. Suppose you are making a solution of a chemical that is 90% pure (that is, 10% of the powder is inert impurities). You need to make a solution that has a concentration of 15 grams of chemical per 500 mL. You need a final volume of 10 L. How much powder must you weight out? (Remember to take into account the inert filler.) Convert 500 mL to 0.5 L 15 grams chemical = X 0.5 L 10 liters X = (15 g chemical)(10 liters) = 300 g chem. 0.5 L But remember that the powder is only 90% pure: 90 g chemical = 300 g chemical 100 g powder X X = (300 g chemical)(100 g powder) = 333 g powder 90 g chemical 2 3. Which powder is more pure: Powder A: Contains 250 mg of NaCl in every 0.3 grams of powder. Powder B: Contains 150 g of NaCl in every milligram of powder. Powder A = 250 mg NaCl / 0.3 g powder Powder A = 0.25 g NaCl = __X__ 0.3 g powder 100 Powder A = 83.3% pure Powder B = 150 g / mg Powder B = 0.15 mg NaCl = __X__ 1mg powder 100 Powder B = 15% pure Therefore, Powder A is more pure.