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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.