1. You will use a 125mL Erlenmeyer flask for the experiment. Describe how you might determine the available volume of the flask. This will be the volume occupied by air or the air+H₂ gas produced in the reaction. Explain how you will account for the following items when you determine the volume of your flask.

1. A 125 mL flask does not have a volume of precisely 125 mL.
2. During the experiment, you will seal the flask with a rubber stopper and the stopper will occupy some of the volume of the flask.
3. You will add 5mL of 3M HCl solution to the flask.

First, we determine the volume of the flask by filling it with water to the brim. Then, we take this volume of water out of the erlenmeyer flask and measure it's volume in a graduated cylinder. Then, to account for the volume the stopper will take up, we will measure the volume of the stopper. We achieve this by putting the stopper in the same graduated cylinder, with the water, and measure the difference in volume from before and after the stopper is put in there. Finally, we account for the solution added. For example, let's say that the flask has 175 mL of space in it. Then we measure the stopper and it takes up 25mL of space. The vessel then would have a volume of \(175\text{mL} - 25\text{mL} - 5\text{mL} = 145\text{mL}\).

2. What does STP mean? Give the values.

STP, in a chemistry context, means Standard Temperature and Pressure. These are 273.15 K and 1 atm.

3. Calculate the volume of H₂ gas, in mL at STP, produced by the reaction of 0.0263 grams of Mg with excess HCl. (Hint: _find moles of Mg, use it to find the moles of H₂ and the volume of H₂_)

The reaction we are to experience can be described by the equation

Mg (s) + 2HCl (aq) -> MgCl₂ (aq) + H₂ (g)

Thus the molar quantity of magnesium in the reactants is the same we should expect as the hydrogen gas after the reaction. Therefore, with 0.0263 grams of magnesium, we can find the molar quantity of hydrogen we should expect using the following formula:

\begin{align*} \text{mol Mg} &= \frac{\text{g Mg}}{\text{molar weight}} \\ &= \frac{0.0263 \text{g}}{24.305 \frac{\text{g}}{\text{mol}}} \\ &= 0.00108 \text{mol} \end{align*}

With this amount of H₂, we can find the volume it should take up using the ideal gas law:

\begin{align*} PV &= nRT \\ V &= \frac{nRT}{P} \\ &= \frac{0.00108 \text{mol} \cdot 8.314462 \text{J}\text{K}^{-1}\text{mol}^{-1} \cdot 273.15 \text{K}} {1 \text{atm}} &= 88.6 \text{mL} \end{align*}

Unit-aware calculators are quite helpful for verifying this. The units work out.

4. What volume will a sample of H₂ gas occupy at 25 degC and 650 mmHg if at STP it occupies 225 mL? (Use eq. given in Calculation Help e)

Using the equation in said section:

\begin{align*} \frac{\text{PV}}{\text{T}}_\text{RTP} &= \frac{\text{PV}}{\text{T}}_\text{STP} \\ \text{V}_\text{RTP} &= \frac{\text{P}_\text{STP} \text{V}_\text{STP} \text{T}_\text{RTP}} {\text{T}_\text{STP} \text{P}_\text{RTP}} \\ &= \frac{1 \text{atm} \cdot 225 \text{mL} \cdot 298.15 \text{K}} {273.15 \text{K} \cdot 0.8552632 \text{atm}} \\ &= 287 \text{mL} \end{align*}

5. At the start of the experiment, before acid is added to the 125 mL flask, what gases does the flask contain? (Three principal gases are present. Use the Internet.)

The three gases that make up most of the atmosphere are diatomic nitrogen, oxygen, and monoatomic argon.