Combustion of a compound of formula CXHYOZ yields 4.169g H20 and 40.74g CO2 when
ID: 878595 • Letter: C
Question
Combustion of a compound of formula CXHYOZ yields 4.169g H20 and 40.74g CO2 when 29.62g of O2 is used. What is the empirical formula of the compound? Note: Entering numbers as subscripts is not required. For example, if the answer is C4H903 enter the answer as C4H903. Enter your answer in the order C, H then O. Answer: There are many minerals which contain silver. A 1.56 g sample of one of these, stephanite, is placed in water and all of its silver ions are released. It takes 2.25 mL of 2.19 M magnesium chloride to precipitate all the silver ions just released as the solid silver chloride. Calculate the mass percent of silver in stephanite to the correct number of significant figures. Assume that stephanite contains no chloride ions.Explanation / Answer
2.19 M means 2.19 mol solute / L
mol MgCl2 used = [2.19 mol MgCl2 / L] x [1 L / 1000 mL] x 2.25 mL
each mol MgCl2 contains 2 mol Cl ions
so mol Cl ions used = [2.19 mol MgCl2 / L] x [1 L / 1000 mL] x 2.25 mL x [2 mol Cl ions / mol MgCl2]
now Ag + Cl AgCl
so the mole ratio Ag : Cl = 1 : 1
so mol Ag present = [2.19 mol MgCl2 / L] x [1 L / 1000 mL] x 2.25 mL x [2 mol Cl ions / mol MgCl2] x [1 mol Ag / mol Cl]
mass Ag in precipitate = [2.19 mol MgCl2 / L] x [1 L / 1000 mL] x 2.25 mL x [2 mol Cl ions / mol MgCl2] x [1 mol Ag / mol Cl] x [107.87 g / mol]
= 1.063 g Ag ... [leaving an extra sig fig to reduce rounding error in the final calculation]
so mass % Ag in stephanite = [1.063 g Ag / 1.56 g stephanite] x 100% = 68.1% Ag