# Stoichiometry Essay

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Chem 1110 - Stoichiometry 1) A 25.00 mL aliquot of H3PO4 was titrated with 43.12 mL of 0.002548 M Ca(OH)2: 2H3PO4(aq) + 3Ca(OH)2(aq) → Ca3(PO4)2(s) + 6H2O(l) What was the concentration of the H3PO4 solution before titration? Express your answer in moles/L. [Ans.: 0.002930 M] 2) In one experiment, 22.9841 grams of 75.25% pure Pb(NO3)2 (pure Pb(NO3)2 has a molar mass of 331.2 grams) was mixed with 51.2354 grams of 81.21% pure CsI (pure CsI has a molar mass of 259.80992 grams): 2CsI(aq) + Pb(NO3)2(aq) → 2CsNO3(aq) + PbI2(s) After the reaction, 10.4025 grams of PbI2(s) (molar mass 461.0 grams) were collected. What is the percent yield of the reaction? [Ans.: 43.21] 3) A 5.6780 gram sample of the compound Na2XO3 was dissolved in water then reacted with excess calcium chloride: Na2XO3(aq) + CaCl2(aq) →CaXO3(s) + 2NaCl(aq) A total of 5.4842 grams of CaXO3 were collected. What is the identity of the element “X?” [Ans.: Se, calculated molar mass 78.93 g] 4) A 0.3528 gram mixture of H2SO4 (molar mass 98 grams) and H3PO4 (molar mass 98 grams) was mixed with a little water and titrated with 9.70 mL of 1.000 M NaOH: 3NaOH(aq) + H3PO4(aq) →3H2O(l) + Na3PO4(aq) 2NaOH(aq) + H2SO4(aq) →2H2O(l) + Na2SO4(aq) What was the percent by mass H2SO4 in the original mixture? [Ans.: 30.6] 5) A 0.9030 gram sample of Z(OH)2 and 20.00 mL of 2.000 M HCl were put into a 100.0 mL volumetric flask and mixed with enough extra water to make 100.0 mL of solution. A 10.00 mL aliquot of this solution was taken and titrated with 17.64 mL of 0.05121 M NaOH. What is the identity of the element Z? The two reactions are: Z(OH)2(s) + 2HCl(aq) →ZCl2(aq) + 2H2O(l) HCl(aq) + NaOH(aq) →NaCl(aq) + H2O(l) [Ans.: Mg, calculated molar mass 24.31 g] 6) A sample of “compound X,” known to contain only C and H, was burned and the resulting CO2 and