rder that,
after precipitating and igniting all the iron to Fe_{2}O_{3}, the
percentage of Fe_{2}O_{4} in the sample may be found by multiplying
the weight in grams of the ignited precipitate by 100?

!Answer!: 0.9665 gram.

89. After oxidizing the arsenic in 0.5000 gram of pure As_{2}S_{3} to
arsenic acid, it is precipitated with "magnesia mixture" (MgCl_{2} +
2NH_{4}Cl). If exactly 12.6 cc. of the mixture are required, how many
grams of MgCl_{2} per liter does the solution contain? H_{3}AsO_{4} +
MgCl_{2} + 3NH_{4}OH = MgNH_{4}AsO_{4} + 2NH_{4}Cl + 3H_{2}O.

!Answer!: 30.71 grams.

90. A sample is prepared for student analysis by mixing pure apatite
(Ca_{3}(PO_{4})_{2}.CaCl_{2}) with an inert material. If 1 gram of
the sample gives 0.4013 gram of Mg_{2}P_{2}O_{7}, how many cubic
centimeters of ammonium oxalate solution (containing 40 grams of
(NH_{4})_{2}C_{2}O_{4}.H_{2}O per liter) would be required to
precipitate the calcium from the same weight of sample?

!Answer!: 25.60 cc.

91. If 0.6742 gram of a mixture of pure magnesium carbonate and pure
calcium carbonate, when treated with an excess of hydrochloric acid,
yields 0.3117 gram of carbon dioxide, calculate the percentage of
magnesium oxide and of calcium oxide in the sample.

!Answers!: 13.22% MgO; 40.54% CaO. 92. The calcium in a sample of
dolomite weighing 0.9380 gram is precipitated as calcium oxalate and
ignited to calcium oxide. What volume of gas, measured over water
at 20 deg.C. and 765 mm. pressure, is given off during ignition, if the
resulting oxide weighs 0.2606 gram? (G.M.V. = 22.4 liters; V.P. water
at 20 deg.C. = 17.4 mm.)

!Answer!: 227 cc.

93. A limestone is found to contain 93.05% CaCO_{3}, and 5.16 %
MgCO_{3}. Calculate the weight of CaO obtainable from 3 tons of the
limestone, assuming complete conversion to oxide. What weight of
Mg_{2}P_{2}O_{7} could be obtained from a 3-gram sample of the
limestone?

!Answers!: 1.565 tons; 0.2044 gram.

94. A sample of dolomite is analyzed for calcium by precipitating
as the oxalate and igniting the precipitate. The ignited product is
assumed to be CaO and the analyst reports 29.50% Ca in the sample.
Owing to insufficient ignition, the product actually contained 8% of
its weight of CaCO_{3}. What is the correct percentage of calcium in
the sample, and what is the percentage error?

!Answers!: 28.46%; 3.65% error.

95. What weight of impure calcite (CaCO_{3}) should be taken for
analysis