# Kinetics: various calculations

1) Calculate the mole fraction of methanol (CH OH) in a mixture of equal masses of methanol and water.

2) Calculate the molar concentration of chloride ions in a solution prepared by dissolving 454g of CaCl in water and diluting to 500mL.

3) At 25 degrees Celcius calcium chloride has a solubility of 60g/100g water, whereas at 100 degrees celcius, the solubility is considerably less. what does this indicate about the dissolutin of calcium chloride in water?

4) The addition of 125mg of caffeine to 100g of cyclohexane lowered the freezing point by 0.13K. Calculate the molar mass of caffeine. k for cyclohexane is 20.1K/mol.

5) What is the molality of a 1.00 Molarity glucose solution with a density of 1.045g/mL.

6) Consider the following reaction

2N O(g)-------→ 2N (g) + O (g)

Rate of decomposition of N O = k[N O]

For an initial concentration of dinitrogen oxide of 0.70M, calculate the concentration of dinitrogen oxide remaining after 3 minutes if k=0.0054 per second.

8) If the rate of a reaction increases by a factor of 2.3 when the concentration of reactant increases by a factor of 1.5, the order of the reaction with respect to this reagent is what?

10) The gas phase decomposition of NaOBr is second order in [NaOBr], with k=0.820 per molar per second at 10 degrees celcius. We start with 0.400M NaOBr in a flask at 10 degrees celcius. How man seconds does it take to use up 0.2015M of this NaOBr?

2NaOBr (g)-------→ 2NO(g) + Br (g)

11) The specific rate constant, k, for a first-order reaction is 0.00916 per second at 0.0 degrees Celsius. The activation energy of this reaction is 88.0 kJ/mol. Determine the value of k at 120 degrees Celsius.

#### Solution Preview

1) Calculate the mole fraction of methanol (CH OH) in a mixture of equal masses of methanol and water.

This is some exercise of your knowledge concerning the composition of solutions.

Mole should indicate you need to convert something to moles. Therefore you will need to be able to find molecular weights of the component materials. Water is easy...18 grams/mole. Methanol is also easy 12 + 16 + 4 = 32 grams/mole.

Now we have equal masses so how many moles for equal masses? Lets just we have 32 grams of methanol and 32 grams of water out of convenience 32 grams methanol is 1 mole. 32 grams of water is 32/18 = 1.7777777778 moles. What is the total number of moles in such a solution - 1 + 1.77777778 = 2.777777778 moles.

And the mole fraction of methanol is 1/(2.77777778) = 0.36

The mole fraction of water is (1-0.36) = 0.64

The mole fraction is independent of the mass you chose in the analysis

2) Calculate the molar concentration of chloride ions in a solution prepared by dissolving 454g of CaCl in water and diluting to 500mL.

Here again you need the molecular weight of CaCl2 and now I find texts with periodic tables helpful, or an online periodic table when the computer is handy, or I have a little plastic card with the periodic table I carry in my wallet for occasions like these. 110.9834980 g/mole from this source: http://www.ch.cam.ac.uk/magnus/MolWeight.html

Now we assume total dissociation of the salt in the solution such that for one mole of salt we have 2 ...

#### Solution Summary

This solution outlines the methods for calculating the mole fractions of gasses. Additionally, this solution covers temperatures and reactions also related to mole fractions.