Chemical dosing

Problem set: 6 worked problems

Each problem is original, authored from the underlying non-copyrightable relationship, with the full worked solution and the exact slip that produces each wrong answer. Work the problem before opening the solution. For a timed, randomized run with a per-topic score, use the free knowledge practice test.

Watch for Dosing stacks two steps: the parts-per-million-to-weight conversion, then dividing by the product's purity. The single most common miss is skipping the purity divisor, which treats a 65 percent product as if it were pure and badly under-doses. Carry the units and the decimal through every line.

1. How much calcium hypochlorite (cal-hypo, 65% available chlorine) raises 30,000 gallons by 2 ppm of free chlorine?

   • A 0.33 lb
   • B 0.05 lb
   • C 0.50 lb
   • D 0.77 lb (about 12.3 oz)
   Show the worked solution

   Correct answer: D. 0.77 lb (about 12.3 oz)

   Pure weight = 30,000 x 2 x 8.34 / 1,000,000 = 0.50 lb. Then divide by the 65 percent purity: 0.50 / 0.65 = 0.77 lb of product. Stopping at 0.50 lb skips the purity divisor and under-doses; multiplying by 0.65 instead of dividing gives 0.33 lb, which is backwards because a weaker product needs MORE weight.

   Source: Public water-chemistry relationship: pure lb = gallons x ppm x 8.34 / 1,000,000, then product lb = pure lb / available-chlorine fraction

2. How much sodium dichlor (56% available chlorine) raises 40,000 gallons by 3 ppm of free chlorine?

   • A 1.00 lb
   • B 0.56 lb
   • C 2.86 lb
   • D 1.79 lb
   Show the worked solution

   Correct answer: D. 1.79 lb

   Pure weight = 40,000 x 3 x 8.34 / 1,000,000 = 1.00 lb. Divide by 0.56: 1.00 / 0.56 = 1.79 lb of dichlor. Leaving it at 1.00 lb skips the purity step; using cal-hypo's 65 percent by mistake would give 1.54 lb, and using lithium hypo's 35 percent gives 2.86 lb, so read the product strength off the label.

   Source: Public water-chemistry relationship: pure lb = gallons x ppm x 8.34 / 1,000,000, then product lb = pure lb / available-chlorine fraction

3. How much sodium bicarbonate raises 25,000 gallons by 20 ppm of total alkalinity? (Sodium bicarbonate has a product factor of 1.679.)

   • A 4.17 lb
   • B 2.48 lb
   • C 0.70 lb
   • D 7.0 lb
   Show the worked solution

   Correct answer: D. 7.0 lb

   Pure weight = 25,000 x 20 x 8.34 / 1,000,000 = 4.17 lb. Sodium bicarbonate is not pure carbonate, so multiply by its product factor 1.679: 4.17 x 1.679 = 7.0 lb. Forgetting the factor leaves 4.17 lb and under-doses; dividing by 1.679 instead of multiplying gives 2.48 lb.

   Source: Public water-chemistry relationship: alkalinity-raising dose = (gallons x ppm x 8.34 / 1,000,000) x product factor; sodium bicarbonate factor 84.0 / 50.04 = 1.679

4. How much cal-hypo (65% available chlorine) raises 15,000 gallons by 1 ppm of free chlorine, expressed in ounces?

   • A About 2.0 oz
   • B About 1.9 oz
   • C About 12.3 oz
   • D About 3.1 oz
   Show the worked solution

   Correct answer: D. About 3.1 oz

   Pure weight = 15,000 x 1 x 8.34 / 1,000,000 = 0.125 lb. Divide by 0.65: 0.193 lb of product. Convert to ounces with 16: 0.193 x 16 = 3.1 oz. Skipping the purity divisor gives 2.0 oz; using 10 instead of 16 for the ounce conversion gives 1.9 oz.

   Source: Public water-chemistry relationship: pure lb = gallons x ppm x 8.34 / 1,000,000; product lb = pure / fraction; 16 oz per lb

5. How much anhydrous calcium chloride raises 30,000 gallons by 10 ppm of calcium hardness? (Product factor 1.109.)

   • A 2.26 lb
   • B 0.28 lb
   • C 2.50 lb
   • D 2.78 lb
   Show the worked solution

   Correct answer: D. 2.78 lb

   Pure weight = 30,000 x 10 x 8.34 / 1,000,000 = 2.50 lb. Multiply by the calcium-chloride factor 1.109: 2.50 x 1.109 = 2.78 lb. Forgetting the factor leaves 2.50 lb; dividing by it gives 2.26 lb.

   Source: Public water-chemistry relationship: hardness-raising dose = (gallons x ppm x 8.34 / 1,000,000) x product factor; anhydrous calcium chloride 110.98 / 100.09 = 1.109

6. How much lithium hypochlorite (35% available chlorine) raises 50,000 gallons by 4 ppm of free chlorine?

   • A 2.57 lb
   • B 1.67 lb
   • C 0.58 lb
   • D 4.77 lb
   Show the worked solution

   Correct answer: D. 4.77 lb

   Pure weight = 50,000 x 4 x 8.34 / 1,000,000 = 1.67 lb. Lithium hypochlorite is weak at 35 percent, so divide by 0.35: 1.67 / 0.35 = 4.77 lb of product. Using cal-hypo's 65 percent by mistake gives 2.57 lb and under-doses; multiplying by 0.35 gives 0.58 lb, which is backwards.

   Source: Public water-chemistry relationship: pure lb = gallons x ppm x 8.34 / 1,000,000, then product lb = pure lb / available-chlorine fraction