SAT II Chemistry : Solutions and States of Matter

Study concepts, example questions & explanations for SAT II Chemistry

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Example Questions

Example Question #1 : Gases

Suppose that an ideal gas at \(\displaystyle 0^{\circ}\text{C}\) and 2.0 atm has a volume of 11.2 cubic meters. Which of the following expressions represents the volume of the gas, in cubic meters, if the temperature were increased by \(\displaystyle 50^{\circ}\text{C}\) and the pressure decreased to 1.5 atm?

Possible Answers:

\(\displaystyle 11.2\text{ m}^3\times\frac{323\text{ K}}{273\text{ K}}\times\frac{1.5\text{ atm}}{2.0\text{ atm}}\)

\(\displaystyle 11.2\text{ m}^3\times\frac{50\text{ K}}{0\text{ K}}\times\frac{1.5\text{ atm}}{2.0\text{ atm}}\)

\(\displaystyle 11.2\text{ m}^3\times\frac{323\text{ K}}{273\text{ K}}\times\frac{2.0\text{ atm}}{1.5\text{ atm}}\)

\(\displaystyle 11.2\text{ m}^3\times\frac{273\text{ K}}{323\text{ K}}\times\frac{2.0\text{ atm}}{1.5\text{ atm}}\)

\(\displaystyle 11.2\text{ m}^3\times\frac{273\text{ K}}{323\text{ K}}\times\frac{1.5\text{ atm}}{2.0\text{ atm}}\)

Correct answer:

\(\displaystyle 11.2\text{ m}^3\times\frac{323\text{ K}}{273\text{ K}}\times\frac{2.0\text{ atm}}{1.5\text{ atm}}\)

Explanation:

First of all, the unit degrees Celsius should be converted to Kelvins by adding 273. This means that the temperature increases from 273 K to 323 K.

Next, the combined gas law can be used.

\(\displaystyle \frac{P_1V_1}{T_1}=\frac{P_2V_2}{T_2}\)

 

Plugging in the known values gives

 \(\displaystyle \frac{2.0\text{ atm}\times11.2\text{ m}^3}{273\text{ K}}=\frac{1.5\text{ atm}\times V_2}{323\text{ K}}\) 

and solving for the volume gives

 \(\displaystyle V_2=\frac{2.0\text{ atm}\times11.2\text{ m}^3\times323\text{ K}}{1.5\text{ atm}\times273\text{ K}}\)

Rearranging shows this is clearly equivalent to the correct answer, \(\displaystyle 11.2\text{ m}^3\times\frac{323\text{ K}}{273\text{ K}}\times\frac{2.0\text{ atm}}{1.5\text{ atm}}\).

Example Question #1 : Effusion

How much faster is the rate of effusion of \(\displaystyle H_2 _{(g)}\) than the rate of effusion of \(\displaystyle H_2O_{(g)}\)?

Possible Answers:

\(\displaystyle 9\)

\(\displaystyle 3\)

\(\displaystyle \frac{1}{3}\)

\(\displaystyle 18\)

\(\displaystyle \frac{1}{2}\)

Correct answer:

\(\displaystyle 3\)

Explanation:

By Graham's Law, \(\displaystyle \frac{Rate_1}{Rate_2}=\sqrt{\frac{M_2}{M_1}}\). The molar mass of \(\displaystyle H_2_{(g)}\) is 2 g/mol and the molar mass of \(\displaystyle H_2O_{(g)}\) is 18 g/mol. Thus, \(\displaystyle \sqrt{\frac{18}{2}}=\sqrt{9}=3\)

Example Question #1 : Concentration And Units

30 mL of 1.0 M  \(\displaystyle HClO_{3}_{(aq)}\)  solution is diluted with water to a volume of 3 L. What is the new concentration of the solution?

Possible Answers:

\(\displaystyle 0.1\textup{\space M}\)

\(\displaystyle 10\textup{\space M}\)

\(\displaystyle 0.01\textup{\space M}\)

\(\displaystyle 0.3\textup{\space M}\)

\(\displaystyle 1\textup{\space M}\)

Correct answer:

\(\displaystyle 0.01\textup{\space M}\)

Explanation:

Recall that \(\displaystyle 1 L = 1000 mL.\) Thus, the volume of the solution increased from 30 mL to 3000 mL, which is a factor of 100. Thus, the new concentration must be 100 times less than the original concentration, and \(\displaystyle \frac{1 M}{100}=0.01 M\).

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