testextracred - PO4-3(aq 3 H(aq = H3PO4(aq Determine the...

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Unformatted text preview: PO4-3(aq) + 3 H+(aq) = H3PO4(aq) Determine the wgt-% of sodium phosphate in the detergent. The number of molecules of phosphate = one-third the moles of hydronium ion. n(phosphate) = (1/3)(0.03900 liter x 0.1500 M) = 0.00195 mole. The mass of sodium phosphate = MW(Na3PO4) x moles of phosphate. MW = (3 x 23.00) + (1 x 31.00) + (4 x 64.00) = 164.00 g/mole. mass of sodium phosphate = (164)(0.00195) = 0.3198 g. The wgt-% of sodium phosphate = (0.3198/0.400)(100) = 80.0% 0.200 g of the detergent is dissolved in 75 ml of water. Determine the molar concentration of phosphate in the resulting solution. Use the results from the first part of the problem. Half the amount of detergent was used so n(phosphate) = (0.5)(0.00195) = 0.000975 mole. The molar concentration of phosphate = moles/volume = 0.000975/0.075 = 0.013 M. A laundry uses hard water which contains appreciable concentrations of calcium ion to dissolve its phosphate detergent. A sales person from Culligan argues that the owner could improve performance by using soft water containing low concentrations of heavy ions such as calcium. Is this claim sales hype or should the owner invest in water softening hardware? Base your answer on chemical principles. The calcium(II) ions in the hard water can precipitate the phosphate as calcium phosphate, thus reducing the concentration of phosphate. More detergent mus be used if a minimum concentration of phosphate is required to do the job. Installing a water softener will eliminate this source of waste and is a worthy investment. (24 points) Sodium phosphate is fairly soluble in water and yields solutions which are moderately basic. Describe the immediate environment around a phosphate anion in an aqueous solution. Six water molecules are arranged around the phosphate anion with the hydrogen atoms, the positive end of the polar water molecule, oriented towards the anion. What can be concluded about HPO4-2 from the above data? Is it a weak or a strong acid? Provide the reasoning for your answer. The solution of phosphate in water is basic so phosphate is a moderately strong base and its conjugate acid, mono hydrogen phosphate, is weak. The acid and base strengths of species are not determined by their solubility. Which is more soluble in an organic solvent such as ether, sodium phosphate or phosphoric acid, a moderately weak acid? Briefly explain. Phosphoric acid is more soluble in ether. Sodium phosphate, an ionic salt and hence polar in the extreme, will be very insoluble in ether, a somewhat non-polar solvent. Phosphoric acid is covalently bound and has a better chance of dissolving in ether as relatively weak intermolecular forces rather than strong intramolecular forces must be broken in order to dissolve. (20 points) Balance the equation for the aqueous-phase reaction which occurs when acidified potassium permanganate is mixed with acetaldehyde (CH3CHO). Under the acidic conditions acetaldehyde is oxidized to acetic acid (CH3COOH) and the permanganate anion is reduced to the Mn(II) cation. 5 e- + 8 H+ + MnO4- = Mn+2 + 4 H2O H2O + CH3CHO = CH3COOH + 2 H+ + 2 e- (five times the top equation, reduction reaction) plus (two times the lower equation) and simplify 5 CH3CHO + 6 H+ + 2 MnO4- = 5 CH3COOH + 2 Mn+2 + 3 H2O (30 points) A chemist is interested in determining the structure of phosphine, a poisonous hydride of phosphorus which is insoluble in water. She reacts an aqueous solution of 0.0050 g of sodium phosphide with an excess of sulfuric acid and collects the gas over water at 23.0 C. She adjusts the leveling bulb of her apparatus so that the pressure in the collection bulb equals atmospheric (barometric) pressure, 735 torr. The stoichiometry for the synthesis is P-3 (aq) + 3 H+ (aq) = PH3(g) Calculate the volume of phosphine (PH3) she collected. The number of moles of phosphine equals the number of moles of sodium phosphide which is given by (0.005 g)/(100.00 g/mole) = 5.0 x 10-5 mole. (100 = 3x23 + 1x31). The partial pressure of phosphine is the total pressure minus the vapor pressure of water, 735.0 torr - 21.2 torr = 713.9 torr = 713.9/760 = 0.939 atm. The Kelvin or absolute temperature of the gas is 273.15 + 23.0 = 296.15 K. The volume is then given by V = nRT/p = (5 x 10-5)(0.08206)(296.15)/0.939 = 0.0013 liter. The gas collected is carefully dried to remove the water vapor and is quantitatively transferred into the cell of a microwave spectrometer which has a volume of 15 liter and has been cooled to 0.0 C. Calculate the pressure of the phosphine in the spectrometer cell. The pressure at the altered temperature and volume is given by p = nRT/V = (5 x 10-5)(0.08206)(273.15)/15 = 7.5 x 10-5 atm. (36 points) Write a balanced, net ionic equation for the reaction which occurs when each pair of aqueous solutions or slurries is mixed. Potassium hydroxide plus acetic acid Acetic acid is a weak acid and potassium hydroxide is a strong base. The hydroxide anion abstracts a proton from the undissociated acetic acid. OH-(aq) + CH3COOH(aq) = H2O + CH3COO- barium carbonate plus cold nitric acid The carbonate anion is a moderately strong base and reacts with 2 moles of hydronium ion from the nitric acid, a strong acid, to produce carbonic acid which decomposes to form gaseous carbon dioxide. Barium carbonate is insoluble and the acid-base reaction provides the drive to dissolve it. BaCO3(s) + 2 H+ = CO2(g) + H2O(l) + Ba+2 chromium(III) sulfate plus ammonia The chromium(III) cation is a transition-metal species and will react with the base ammonia, a ligand, to form a transition-metal complex. Cr+3(aq) + 6 NH3(aq) = Cr(NH3)6+3(aq) sodium iodide plus lead(II) nitrate Halide ions are normally soluble but halide salts of lead(II) are anomalous and are insoluble; a precipitate is formed. 2 I-(aq) + Pb+2(aq) = 2PbI2(s) ...
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This note was uploaded on 07/21/2011 for the course CHEM 111 taught by Professor Sawyers during the Spring '11 term at Virginia College.

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