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"Good morning. Please complete the following quiz over all the material we have covered to date. Once that is finished, please take this handout on meiosis and study it for next class. Once you are finished with your quiz, you may leave."

The students may notice Ms. Sidle not looking precisely angry, but definitely distracted.

"Good morning. I hope you all have been studying the handouts I gave over Dalton's law. Today, we will be doing a series of calculations."

Calculations with Dalton's Law:
Let's try that last experiment with real numbers. In our lab, the atmospheric pressure is 102.4 kPa. The temperature of our water is 25°C. We used a 250 mL beaker instead of a test tube to collect the hydrogen. Let's find the pressure of the hydrogen, and then find the moles of hydrogen using the ideal gas law.

Step 1: We need to know the vapor pressure of the water. A common table lists the pressure at 25°C as 23.76 torr. A torr is 1 mm of mercury at standard temperature. In kilopascals, that would be 3.17 (1 mm mercury = 7.5 kPa). We should also convert the 250 mL to .250 L and 25°C to 298 L.

Step 2: We can use Dalton's Law to find the hydrogen pressure. It would be:
PTotal = PWater + PHydrogen
102.4 kPa = 3.17 kPa + PHydrogen

So the pressure of Hydrogen would be: 99.23 kPa or 99.2 kPa.

Step 3: We use the Ideal Gas Law to get the moles. Recall that the Ideal Gas Law is:
PV=nRT

where P is pressure, V is volume, n is moles, R is the Ideal Gas Constant (0.0821 L-atm/mol-K or 8.31 L-kPa/mol-K), and T is temperature.

Therefore, our equation would be:
99.2 kPa x .250 L = n x 8.31 L-kPa/mol-K x 298 K

This can be re-arranged so:
n = 99.2 kPa x .250 L / 8.31 L-kPa/mol-K / 298 K
n = .0100 mol or 1.00 x 10-2 mol Hydrogen

Another important contribution by John Dalton was his generalization that all gases expand equally on going to the same higher temperature.

"Before we continue with our study on blood splatter, I'm conducting a review session in the metric system of measurement, so everyone is up to speed."

She passes around a series of worksheets. "Please be certain you are familiar with all of these by Friday. There will be a quiz and we will continue with our blood analysis. You can go."

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