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PRE-LAB QUESTIONS

1. Why is this investigation carried out in a basin of ice water that is at or near 0-degrees Celsius (°C)?- This experiment is carried out in a basin of ice water at the given temperature because

it calls for one to confirm the accepted molar volume of 22.4 liters per mole at the standard temperature and pressure, which are 0°C and 760 mmHg (1 atm). By setting the conditions of the experiment in this way, one can easily determine the desired measurement of volume.

2. Write the balanced equation for the reaction that occurs between magnesium and hydrochloric acid.- Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2 (g)

3. How is the volume of gas collected at room temperature adjusted to standard pressure?- In order to adjust the volume of collected gas to standard pressure, one must use the

combined gas law P1V1/T1 = P2V2/T2. However, since the temperature is constant throughout the entire experiment one can simplify the equation to P1V1 = P2V2. P1 represents the atmospheric pressure at room temperature (mmHg), V1 represents the volume at the same pressure (L), P2 equals 760 mmHg and V2 is the value that must be determined.

4. What happens to the acid that is placed in the graduated cylinder at the start of the experiment? How might this pose a safety hazard?- At the start of the experiment, the acid is placed in the graduated cylinder before the

water. This is done in the exact order mentioned because it prevents a reaction from occurring. Pouring the water in before the acid may trigger the reaction, posing a safety hazard. Additionally, the cylinder must be placed cautiously into the basin of ice water to prevent the acid from spilling onto the floor or onto someone’s body.

OBSERVATIONS

DATA TABLE

TRIAL 1 TRIAL 2Length of Mg strip (cm) 1 cm 1 cmVolume of hydrogen collected (mL)

8.3 mL 8.2 mL

Temperature of water (°C) 1.2 °C 1.1 °C

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Atmospheric pressure (mmHg)

768.35 mmHg 770.64 mmHg

CALCULATIONS

Mass of Magnesium Strip

1.00 m (or 100 cm) has a mass of 0.91 g

1.00 cm has a mass of 0.0091 g

Moles of Magnesium

Calculate # moles Mg: 0.0091 g×mol Mg

24.305gMg=0.000374molMg

Moles of Hydrogen Gas

Moles H2 = Moles Mg

0.000374 mol H2

Volume of Hydrogen Gas Collected

8.2 mL H2 collected = 0.0082 L H2 collected

Atmospheric Pressure

P1=Patm

Patm = 30.34 inHg

Patm in mmHg = 30.34 inHg×25.4mm

1∈¿=770.64mmHg ¿

Boyle’s Law

P1V 1=P2V 2 (Boyle’s Law to account for atmospheric pressure not being 760.00 mmHg)

770.64mmHg×0.0082L=760mmHg×V 2

V 2=0.00831L

Molar Volume of Hydrogen Gas

V 2

mol H 2

= Lmol

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0.00831 L0.000374mol

=22.22L/mol

CRITICAL THINKING: ANALYSIS AND CONCLUSIONS

1. Based on your data, calculate the molar volume of hydrogen (liters H2/moles H2) and compare to the accepted value of 22.4 L/mole by computing the percent error.- The percent error is computed by making the following calculations:

Percent Error= experimental value−accepted valueaccepted value

×100 %

22.22−22.422.4

×100%=−0.804 %

2. How would you explain the percent error for your calculation of molar volume?- A source of error that could’ve possibly affected the percent error for this calculation

included ignoring the temperature during the experiment. The experiment called for the water to be set at 0°C, but it was actually set at 1.1°C. Also, failing to measure the volume of hydrogen gas at the exact moment the magnesium strip completely finished dissolving could contribute to the percent error. Lastly, hydrogen gas may have either escaped through slight openings in the sides of the plastic wrap or surrounding air may have entered through the same way.

3. How would you redesign this experiment to reduce the percent error?- In order to reduce the percent error, one could set the temperature of water to a

temperature at or extremely close to 0°C. The closer the temperature is to the desired conditions, the more accurate the calculations will be. Also, parafilm may serve as an appropriate replacement for the plastic wrap used in the experiment. Using this will prevent surrounding air from entering or hydrogen gas from escaping.

4. Why do you think the graduated cylinder was inverted in the ice water bath rather than being held right side up?- Holding the graduated cylinder in this position allowed the hydrogen gas to rise and

isolate itself from the reacting substances. Water has a heavier density than that of the gas, so holding the cylinder right side up would trap the gas rather than release it. By holding it upside down, the gas was released efficiently.

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5. What type of chemical reaction- single replacement, double replacement, decomposition, or direct combination- occurred between the magnesium and the hydrochloric acid?- A single replacement reaction occurred between the magnesium strip and the

hydrochloric acid. This is evident in the balanced equation, which shows the magnesium replacing the hydrogen and ultimately forming magnesium chloride and hydrogen gas.

CRITICAL THINKING: APPLICATIONS

1. How do you think each of the following factors would affect the accuracy of your results when compared with the accepted volume of 22.4 liters per mole at 0°C and 760 mmHg?

a. Gas temperature higher than 0°C – Increasing the temperature of the gas would increase its volume as well, resulting in less-accurate results. This is so because the temperature and volume are directly proportional to each other.

b. Evaporation of water vapor into the collected hydrogen- The evaporation of water vapor into the hydrogen gas would also increase the volume, as more gas is being collected. Similar to the situation above, this would form less-accurate results.

c. Using a longer strip of magnesium- This situation is similar to the above two in that it also increases the volume of hydrogen gas. Using more magnesium would mean having more magnesium to react with HCl. Therefore, more hydrogen gas will form, the volume will increase, and the accuracy of results will decrease.

2. Since gases change volume with changes in pressure and temperature, explain why it is not immediately obvious how much gas is in a propane gas tank. What is the minimum information you need to compare suppliers of propane gas for refilling barbecue propane tanks?

- It is not immediately obvious how much gas is in a propane gas tank because tanks are filled by weight rather than volume. When changes in pressure and temperature are present, the gas’s volume tends to fluctuate in accordance to its conditions. In order to compare suppliers of propane gas, one would only need to know the size of empty tanks and their weights. The amount of liquid and gas propane would also have to be determined because they have different volumes in the same conditions.