Question

8. What is the molar fraction of benzene in a solution made by mixing 10 g of benzene (Molecular Weight 78.11) and 10 g of toluene (Molecular Weight 92.14)? Show two significant digits in your answer.

9. Radioactive Iodine-131, with a half-life of eight days, is a common fission product and thus is present in high levels in radioactive fallout. How long will it take for initial radiation intensity to drop 8-fold?

10. In an experiment, a sample was 90% decomposed in 24 min. Approximately how long would this decomposition have taken if the sample had been heated 20°C higher? (Hint: Assume the rate doubles for each 10°C rise in temperature.)

11. Find which kind of catalyst is necessary to facilitate the reaction between molecular nitrogen and molecular hydrogen to make ammonia (several words description)

Answer 1

8. What is the molar fraction of benzene in a solution made by mixing 10 g of benzene (Molecular Weight 78.11) and 10 g of toluene (Molecular Weight 92.14)? Show two significant digits in your answer.

moles of benzene = mass/ molar mass = 10g/78.11g/mol = 0.1280 mol

moles of toluene = mass/molar mass = 10g/92.14g/mol = 0.1085 mol

mole fraction of benzene = moles of benzene /total moles = 0.1280 /(0.1280+0.1085) = 0.54 12

9. Radioactive Iodine-131, with a half-life of eight days, is a common fission product and thus is present in high levels in radioactive fallout. How long will it take for initial radiation intensity to drop 8-fold?

half life = 8 days

radiation intensity = rate of disintegration

As the rate of disintegration(intesity of radiation) is proprtional to the concentration of the material

initial moles/concentration =[A]o

and after dropping 8-fold concentration = [A]o /8

We know that for radioactive decay

[A] at time t = [A]o /2n where n is the number of half lifes

[A]o /8 = [A]o /2n or 2n = 8

Thus n = 3 (as 8=23) and in 3 half lives time the material reduces 8-fold and the radioactive intensity drops 8-fold.

Time required = 3 half lifes = 3x 8days = 24 days.