# The molar mass of aluminum by measuring the amount of hydrogen gas liberated when a sample of aluminum is treated with excess hydrochloric acid

This experiment involves the determination of the molar mass of aluminum by measuring the amount of hydrogen gas liberated when a sample of aluminum is treated with excess hydrochloric acid.

1. In the reaction between aluminum and hydrochloric acid, how many moles of hydrogen gas would be liberated when 32.26 miligrams of aluminum are treated with excess hydrochloric acid?

40.32E-2 mol

2. The atmospheric pressure was measured to be 0.9247 atm and the room temperature was 19.00oC. At this temperature, the vapor pressure of water is 16.50 torr. What would be the volume of the number of moles of hydrogen just calculated under these conditions? in mL.

3. The syringe used in this experiment is calibrated to 60 mL. However, there is a 'dead volume' at the needle end of the syringe that is uncalibrated. This volume equals 1.20 mL. Bearing in mind that the syringe is upside down, what would the reading on the syringe be that corresponded to the volume you have just calculated?

4. Suppose that you have completed this week's experiment and have determined that the molar mass of aluminum is 22.35 g mol-1. What is the percent error of your experimental result?

• There is only need to invoke the ideal gas law for question 2. Simple stoichiometry calculations should suffice for 1. Try again to see if you can do them now! – Nicolau Saker Neto Nov 3 '13 at 23:38
• it should yield 40.32mL; but how to measure that precisely I dont know – titus Nov 3 '13 at 23:43

I think the friction in the syringe won't make the measurement precise.
I'm no chemist but this is an idea:
Put the aluminium in an erlenmeyer flask.
Find a hose and plug to put on top of the erlenmeyer flask.
Make the hose if a U shape and pour some water into it like this.
Mark the hose to have a reference where the water is.
Add HCl to aluminium and put the plug+hose on top really fast.
The hydrogen will push on the water in the U turn and make it unequal.
Measure the inner diameter of the hose and the length of hose from the reference to where the water is now.
There is some pressure created because the water is unequal in the hose.
Measure the difference in height between the two columns of water. Figure out the pressure given the given column of water with this.
Use PV=nRT to compute n.
The reaction is exothermic so T might not be room temperature.

or

Put the acid and the aluminium in the syringe, plug the end of the syringe and see how much it expands. Although you wouldn't want a syringe under pressure with HCl in it. Also make sure the syringe is larger than 40mL, more like 50mL.