cium sulphate. How much calcium carbonate and strontium carbonate were contained in the mixture? Ans. 0.7554 g. CaCO3 0.3167 g. SrCO3 481. 0.5541 g. of a mixture of potassium iodide and bromide yield 0.3396 g. of potassium sulphate. How much iodine and bromine in the mixture? Ans. 0.2436 g. I 0.1581 g. Br 482. A sample of a mineral containing sodium bromide and sodium chloride weighing 17.4000 g. was dissolved and made up to one liter. Of this solution, 50.00 c.c. when treated with silver nitrate gave 2.0050 g. of mixed silver bromide and silver chloride. The mixed silver halides on treatment with chlorine produced 1.9100 g. of silver chloride. Calculate the percentages of sodium bromide and sodium chloride in the original sample. Ans. 75.18% NaCl 25.27% NaBr 483. A sample containing only pure calcium carbonate and pure magnesium carbonate weighing 2.0226 g. produced 516 c.c. of carbon dioxide measured moist at 20° C. and 750 mm. What are the percentages of MgO and CaO in the sample? The tension of aqueous vapor at 20° is 17.54 mm. One liter of CO2 at standard weighs 1.9654 g. Ans. 48.83% CaO 6.15% MgO 484. A mixture of potassium sulphate and sodium sulphate weighs 1.4304 g. From this mixture 2.1364 g. of barium sulphate are obtained. (a) What are the weights of potassium sulphate and sodium sulphate in the mixture? (b) If 1.5000 g. of the substance were taken from which this mixture was obtained, what are percentages of sodium and potassium in the original sample? Ans. (a) 0.7252 g. Na2SO4 0.7052 g. K2SO4 (b) 15.65% Na 21.10% K 485. Calculate the factors that applied to the previous problem (484) will give the weights of potassium and sodium. These are to be 486. One gram of a mixture consisting of silver chloride and silver bromide is found to contain 0.6635 g. of silver. (a) How much bromine and (b) how much chlorine does it contain? Ans. (a) 0.2129 g. Br (b) 0.1236 g. Cl 487. Calculate factors which when multiplying the weight of the mixed salt of problem 486 and the weight of silver obtained from the mixture will give the weights of bromine and chlorine present. 488. A mixture of barium and strontium carbonates weighing 1.2601 g. contains 0.3192 g. of carbon dioxide. (a) What are the weights of barium carbonate and strontium carbonate in the mixture? (b) What are the weights of barium and strontium in the mixture? Ans. (a) 0.7504 g. BaCO3 0.5097 g. SrCO3 (b) 0.5225 g. Ba 489. Calculate the factors that applied to a problem such as 488 will give the weights of barium and strontium. 490. A sample of oleum weighing 3.1402 g. contains 2.7350 g. of sulphur trioxide. What per cent oleum is it? Ans. 29.78% oleum. 491. What is the per cent total sulphur trioxide in 25% oleum? Ans. 86.22% 492. What is the per cent total sulphur trioxide in 15% oleum? Ans. 84.39% 493. If acetic anhydride (CH3CO)2O will take up any water present to form acetic acid, what is the amount of uncombined acetic anhydride present in a sample of mixed acetic acid and acetic anhydride showing 87.72% anhydride present? Ans. 18.13% CHAPTER IX VOLUMETRIC ANALYSIS "VOLUMETRIC analysis or quantitive chemical analysis by measure in the case of liquids and solids . . . depends upon the following conditions for its successful practice: 1. A solution of the reagent, the chemical value of which is accurately known, called the 'standard solution.' 2. A graduated vessel from which portions of it may be accurately delivered, called the 'burette.' 1 3. The decomposition 1 produced by the standard solution with any given substance must either in itself or by an indicator be such, that its termination is unmistakable to the eye, and thereby the quantity of the substance with which it has combined accurately calculated." 2 Single-factor Solutions. - The standard solution may be made to any desired strength. If a solution is to be frequently used for the determination of a single substance, for example iron, it is convenient to adjust its strength so that one cubic centimeter shows the presence of 0.01, 0.001 or 0.0001 g. of iron, in which case the determination of the weight of iron in grams present in the sample is obtained directly by the burette reading. If it is desired to make up a solution of silver nitrate, one cubic centimeter of which is to be equivalent to 0.001 g. of chlorine, and the weight of silver nitrate which must be contained in a liter of the solution is to be calculated, the required weight can be found readily. The reaction is M'Cl + AgNO3 = M'NO3 + AgCl. The weight of chlorine which is to be indicated by one liter is 0.001 X 1000. = 1.0000 g. The amount of silver nitrate necessary is It commonly occurs that a substance with which it is desired to make a standard solution cannot be weighed out in a state of purity; hence it is necessary to standardize the solution after it is made up. The methods of doing this are many: typical cases will be considered. 1 "Reaction" might be a better word. 2 Sutton, "Volumetric Analysis." Methods of Standardization. I. By weighing out the amount required and making up to the desired volume. This can be done only when the substance is in a state of purity and the weighing can be accurately performed and presents no great difficulties in manipulation. Among such substances may be mentioned sodium carbonate, arsenious oxide and sodium oxalate. II. By causing a solution to react with a weighed amount of a chemically pure substance and measuring the volume of the solution necessary to do this. o.2119 g. of anhydrous sodium carbonate require 40.00 c.c. of a solution of hydrochloric acid to completely neutralize it. The strength of the hydrochloric acid per cubic centimeter is III. By analyzing gravimetrically a measured portion of the solution. If 30.00 c.c. of the same solution of hydrochloric acid as above yield 0.4299 g. of silver chloride, the content of hydrochloric acid per cubic centimeter is IV. By comparing a measured volume of the solution with a measured volume of another solution the strength of which is known and with which it reacts. If 45.00 c.c. of the same hydrochloric acid solution react with 45.20 c.c. of a solution of sodium hydroxide, each cubic centimeter of which contains 0.003981 g. of sodium hydroxide, the strength of the hydrochloric acid solution is Equivaler+ Values of Single-factor Solutions. A solution may be made up to a strength to correspond to a simple factor, as, for instance, the solution of silver nitrate mentioned above, each cubic centimeter of which corresponds to a milligram of chlorine. When such a solution is to be used for another determination the value of the solution per cubic centimeter must be calculated for the new substance. Potassium permanganate is a reagent used in a number of different volumetric determinations. Its value depends upon its oxidizing property. Iron is determined by it according to the equation 2KMnO4 + 10FeSO4 + 8H2SO4 = K2SO4 + 2MnSO4 + 5Fe2(SO4)3 + 8H2O; oxalic acid, 2KMnO4 + 5H2C2O4 + 3H2SO4 = K2SO4 + 2MnSO4 calcium when in the form of the oxalate, 2KMnO4 + 5CaC2O4 + 8H2SO4 = K2SO4 + 2MnSO4 manganese, 2KMnO4 + 3MnSO4 + 2H2O = K2SO4 + 2H2SO4 There are many other similar reactions. If one cubic centimeter of the potassium permanganate solution represents 0.001 g. of iron, then as 2KMnO4 = 10Fe, and 2KMnO4 = 5H2C2O4, it follows that 5H2C2O4 = 10Fe: the amount of oxalic acid equivalent to one cubic centimeter of this permanganate solution is: 5H2C2O4 5(90.016) X 0.001 = 0.000806 g. H2C2O4 per c.c. In like manner, 2KMnO4 = IoFe; 2KMnO4 = 5Ca; then 5Ca = IoFe, and the amount of calcium indicated by each cubic centimeter of the permangante solution is 5Ca 5(40.07) = IoFe 10(55.84) X 0.001 = 0.0003588 g. Ca per c.c. Finally, as 2KMnO4 = 10Fe and 2KMnO4 = 3Mn, it follows that 3Mn = 10Fe, and the value of the permanganate solution in terms of manganese is 3 Mn = 3(54.93) 10(55.84) X 0.001 = 0.0002951 g. Mn per c.c. Factor Weights for Volumetric Solutions. Factor weights1 are as applicable to volumetric as to gravimetric analysis. Acetic acid is to be determined by titration with sodium hydroxide, one cubic centimeter of which contains 0.04006 g. NaOH. What weight of acetic acid should be taken such that the burette reading in cubic centimeters may show the percentage of acetic acid directly? One cubic centimeter of the sodium hydroxide solution is equivalent to |