Homework Help: Chemistry

Posted by Jake on Wednesday, March 11, 2009 at 11:17pm.

I'm about to do the second part of a Limiting Reagent laboratory tomorrow. There are a few questions that are giving me trouble on the pre-lab assignment.

In the first lab period, we measured the amount of H2 gas produced in a reaction of Mg and 10 mL of 2.0 M HCl. We varied the mass of Mg, starting at 0.050 g and increasing by increments of 0.050 g up to 0.400 g.

With the data collected, we constructed a graph of volume of H2 gas produced (mL) vs. mass of Mg (g). We had to include a best fit line in the linear portion of the graph.

An analysis of the graph shows that as the mass of Mg increases, the volume of gas increases proportionally until Mg is no longer the limiting reagent. Once 2.0 M HCl becomes the limiting reagent, the graph becomes almost vertical since this amount of H2 gas is the maximum amount that can be produced in the reaction.

What I'm required to do is this:

Using your graph,

*** 1. Determine the value R1 (volume of H2 gas produced per gram of Mg that reacts).

*** 2. Using the value of R1, determine the value R2 (volume of H2 gas produced per mole of Mg that reacts).

*** 3. At a given temperature and pressure, the volume of a gaseous sample is directly proportional to the moles of gas (Volume = C * moles, where C is a constant). Assuming that the temperature and pressure is the same for the reaction of magnesium with HCl and zinc with HCl and considering the balanced chemical equations for these reactions, answer the following question: What is the relationship between the factor R2 and the factor R3 (volume of H2 gas produced per mole of zinc that reacts)?

The balanced chemical equations are:

Mg + 2HCl --> H2 + MgCl2
Zn + 2HCl --> H2 + ZnCl2

Chemistry - DrBob222, Thursday, March 12, 2009 at 12:13am
If I interpret the question correctly, for #1, you can read the volume vs grams Mg. Convert that to volume vs 1 g Mg. A simple proportion will do it.
2. Same thing for #2 EXCEPT change the 1 g to 24.32 g (I think that's the atomic mass of Mg). I wouldn't make a straight multiplication of what I had found for 1 g because of rounding errors but read from the graph and convert to the molar mass from the mass you read.
#3. Do that for Zn. The volume/mole should be the same (but the volume/gram will not be the same).
Chemistry - Jake, Thursday, March 12, 2009 at 12:21am
For example, if I pick a point at random on the graph.. say (0.200, 184.3).

Then
R1 = volume H2 / mass Mg
R1 = 184.3 mL H2 / 0.200 g Mg
R1 = 921.5 mL H2 / g Mg

for the next part it says "using the value of R1" so...

R2 = (184.3 mL H2 / 0.200 g Mg) * (1 mol Mg / 24.31 g Mg)
R2 = 37.90621144 mL H2 / mol Mg
R2 = 37.91 mL H2 / mol Mg

then do essentially the same thing for R3 and make the comparison between R2 and R3.
Chemistry - Jake, Thursday, March 12, 2009 at 12:23am
would i get a more accurate answer using the slope instead of a point at random?
Chemistry - DrBob222, Thursday, March 12, 2009 at 1:36am
Won't the line be a straight line as long as you haven't reached the limiting reagent part. I think you want the point. You may want to pick two or three points and use the average.

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