Email questions...

A few questions have arrived in my email box. Don't forget to check the answer keys that are posted in my mnstate.edu web page. Here are the questions...

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Practice set one:
- on #4, I know that it is a three-step question. However, I can't get from 100C to 136.19. The H/fusion H/vaporization has me confused.
- #5 and 6, I don't know how to set them up. I'm sure I can figure it out, but I don't know from where to start.

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On #4, you need to do a heat capacity problem to heat the liquid water up to 100degC, then a {delta}H(vaporization) problem to convert all the liquid water to gaseous water, then another heat capacity problem to get from 100degC to 136.19degC. I think when I wrote the answers up on the board I had a math error in the steam part, check the answer key for the correct numbers.

For #5 and #6, start with the reactant or product that you have enough information about to calculate the rate, then figure out the rate with respect to the other reactants and products using the stoichiometry of the balanced chemical equation. For example, in #5 you react 3 mols of hydrogen gas for each mol of nitrogen gas, so the rate of hydrogen consumption should be 3x the rate of nitrogen consumption.

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Practice set two:
- #1, I don't understand what to do with the grams and how to make it into a reaction order.

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Rate laws relate concentrations to rates, so you will (at some point) need to calculate initial concentrations based upon the grams of starting materials given. You can actually figure out the orders of the rate law expression using grams, but to get a proper value of "k" with concentration units, you'll need to convert.

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I'm not very good at conversions and I am stumped on how you would convert mL or L to kg?..for problems like finding molality? In particular question 2 on the first problem set?

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Volume and mass are related to one another by the density of a substance. Densities are often reported in units of "g/mL", so work through whatever unit conversions you need to get through the density. For example, the density of chloroform is 1.5 g/mL, so if I want to know the mass of 2.0L of chloroform in kg...

(2.0L)(1000mL/1L)(1.5g/1mL)(1kg/1000g) = 3.0kg

As with most of the problems we'll work with, if you're having trouble getting things set up correctly, make sure you keep an eye on the units. It won't always be magic, but you'll have a better chance of setting things up correctly if your units work out.

Good luck and let me know of there are other questions.