Question

2. In the lab, you grind up 2.205 g of an unknown compound and dissolve it in 250 mL of water. You take 5 mL of this solution and dilute it to a volume of 125 mL. Using a titration, you measure the concentration of the resulting solution to be 1.96*10-3 M. Determine which of the following could be the formula of the compound. Show your work. You will need to work backwards and remember that the molar mass is equal to the mass of a sample over the number of moles in it.

N2O4   P4O10   C6H12O6 MnO2 UF6

Answer 1

Given data: 2.205g /250 mL of water

Conc: 8.82 g/1000mL = 8820 mg/1000 mL = 8.82 mg/mL

5 mL contains = 8.82 mg/mL x 5 mL = 44.1 mg

Diluted to 125 mL

So the conc. of the solution is 44.1 mg/125 mL = 0.3528 mg/mL

So the conc. is 0.352 g/L

Conc : 1.96 x 10^-3 M (by Titration)

N₂O₄: 92.011 g/mol = 0.352/92.011 = 3.82 x 10^-3 M

P₄O₁₀: 92.011 g/mol = 0.352/283.89 =1.24 x 10^-3 M

C₆H₁₂O₆: 180.155 g/mol = 0.352/180.15 = 1.95 x 10^-3 M

MnO₂:86.93 g/mol = 0.352/86.93 = 4.04 x 10^-3 M

UF₆: 352.02 g/mol = 0.352/352.02 = 9.9 x 10^-4 M

So, the formula of compound is C₆H₁₂O₆