A mixture of aluminum and zinc metal is used in 2 experiments described below:
A- A 1.7 gram sample of the mixture is reacted with excess hydrochloric acid and the resulting hydrogen gas is collected over water. The reaction takes place at 750 mm Hg atmospheric pressure and 25C. VP of water = 23.8 torr.
B- In a separate experiment, 1.4 grams of the mixture is reacted with a stoichiometric quantity of hydrochloric acid. The resulting solution is treated with a concentrated ammonia solution. A precipitate forms that is collected and dried. The mass of the precipitate is 2.70 gms.
Calculate the volume of the hydrogen gas collected in A.
Any help would be appreciated, even if it is just where to start.
Im mainly confused on how you know what amount of aluminum or zinc is in the samples of the mixture
AP Chemistry - DrBob222, Sunday, April 3, 2011 at 2:49pm
You find that from part B. The reaction in part B is
Zn + 2HCl ==> ZnCl2 + H2
2Al + 6HCl ==> 2AlCl3 + 3H2
At this point you have Zn^+2 ions and Al^+3 ions in solution. Treatment with concd NH3 solution forms Al(OH)3 with the aluminum ions and Zn(NH3)4^+2 with the zinc ions. So convert 2.70 g of the ppt formed [Al(OH)3] to grams Al and subtract from the total to arrive at the grams Zn. I would get a percent at this point. Then go back to part A, use the percentage to find the amount Zn and Al in the 1.7 g sample, then through the stoichiometry to find the moles H2 from Zn and moles H2 from Al, then use PV = nRT to solve for volume in liters.
AP Chemistry - Kate, Sunday, April 3, 2011 at 3:41pm
Where does the Al(OH)3 come from and how do you know and how do you tell how much of the 2.70g of ppt is Al(OH)3 or Zn(NH3)4^+2??
Thanks
I just finished answering your follow up question below. See the original post.
In order to calculate the volume of hydrogen gas collected in experiment A, you need to determine the amount of aluminum and zinc present in the mixture.
Based on the information provided, we can start by looking at experiment B. In this experiment, 1.4 grams of the mixture is reacted with hydrochloric acid to form a precipitate. The balanced chemical equations show that when zinc reacts with hydrochloric acid, zinc chloride and hydrogen gas are produced. Similarly, when aluminum reacts with hydrochloric acid, aluminum chloride and hydrogen gas are produced.
Now, we know that the precipitate formed in experiment B is either aluminum hydroxide (Al(OH)3) or zinc ammine complex (Zn(NH3)4^+2). Through a series of chemical reactions, the aluminum ions (Al^+3) react with the concentrated ammonia solution to form the precipitate of aluminum hydroxide, while the zinc ions (Zn^+2) form the zinc ammine complex.
To determine the amount of aluminum and zinc in the precipitate, you can convert the 2.70 grams of the precipitate (Al(OH)3) to grams of aluminum by assuming that the entire precipitate is aluminum hydroxide. Then, subtract this amount from the total amount of the mixture in order to find the amount of zinc.
Now that you have determined the amount of aluminum and zinc in the mixture, you can use this information to calculate the moles of hydrogen gas produced in experiment A. For example, if you have moles of aluminum and moles of zinc, you can use the balanced chemical equations to find the moles of hydrogen gas produced from each component. Finally, you can use the ideal gas law equation, PV = nRT, where P is the pressure (750 mm Hg), V is the volume of hydrogen gas (which is what we are trying to find), n is the number of moles of hydrogen gas, R is the ideal gas constant, and T is the temperature (25C converted to Kelvin), to solve for the volume of the hydrogen gas in liters.