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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: invamous on March 14, 2009, 05:30:54 PM

Title: Basic mole questions
Post by: invamous on March 14, 2009, 05:30:54 PM

 Recently in my chemistry class, we have been learning about moles. I think I have a pretty good grasp on the concepts, but I would appreciate it if you guys would check my work on an extra credit problem, because I'm not sure if I correctly understand it. Thanks.  :)

 Question: A water solution of sulfuric acid has a density of 1.67g/ml and is 75 percent H2SO4 by weight. How many moles of H2SO4 are contained in 300 ml of this solution?

 My answer: 3.834 moles

First, I multiplied 1.67 by 300 to figure out the mass in grams of the solution.

1.67g/ml x 300ml = 501 grams
                 
Then I multiplied 501 by .75 to determine the mass of the sulfuric acid. 501 grams x .75 = 375.75 grams

Since the formula weight of sulfuric acid is 98, I divided 375.75 by 98, and that equals 3.834 moles.


 Okay, so that's it. If my method for solving the problem was incorrect, or I had a mathematical error, please explain to me where I went wrong. Once again, thank you in advance.

Title: Re: Basic mole questions
Post by: Arkcon on March 14, 2009, 06:20:25 PM

Here's a trick to help you be sure you've done the calculations correctly.  Put the units of your known values in you calculations.  If the units cancel out, you know you've divided when you should have.  When you get the correct units as an answer, you'll know that you have it right.  For example:  density = mass / volume.  You multiplied density by volume to get mass, as you can see, that is correct.  Next time, when you have mass, and need volume, for example, you'll be able to see what to do.