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Concentration and Molarity Help (page 3)

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By — McGraw-Hill Professional
Updated on Aug 28, 2011

Concentration

Sometimes an experiment requires a weaker acid solution than what a chemist has on the shelf. In order to do the experiment, the solution must be diluted. This is done by figuring out the molarity and volume of the solution.

n d = the number of moles in the dilute solution

v d = the volume of the solution

M d = the molarity of the solution

To figure out the number of moles of solute in the dilute solution, use

M d × v d = n d

To figure out the number of moles of solute in the concentrated solution, use

M c × v c = n c

The trick is that the number of moles does not change.

Moles of solute = n d = n c

So the equation looks like M c × v c = M d × v d

M d = M c × v c / v d

As you begin working in a chemistry lab, you will use this formula more. For now, just an understanding of the relationship between solute and solvent is enough.

Dilution

Solutions can be found in their concentrated forms in many laboratories. They have a variety of uses, but, in general, it is easier to keep one concentrated solution on hand than five dilutions of the same solution. There is just not that much space on the shelves. It is important to learn how to make a less concentrated solution from a concentrated one. Look at the following example.

Examples

Example 1

What would you do if an experimental procedure called for 1 M of hydrochloric acid (HCl) and all you had in the lab was 12 M HCl? Could you use what you had on hand? Sure! Just prepare the 1 M HCl by measuring a volume 1/12 or 82 milliliters of the concentrated solution into 1 liter of distilled water. The final concentration is equal to 1 M HCl.

Example 2

How about nitric acid (HNO 3 )? What if you needed 1 M of nitric acid for an experiment and only had concentrated nitric acid (16 M) on hand? You would measure out 63 ml of the concentrated HNO 3 , then add enough distilled water to equal 1 liter. The resulting solution would be equal to 1 M HNO 3 . If you needed a 3 M solution, multiply the 63 ml by 3 to get 189 ml to add to water to bring it to 1 liter. The resulting solution would be a 3 M solution.

To make diluted solutions, chemists use volumetric flasks or beakers for accurate measuring. With a bit of practice, making dilutions of concentrated solutions right off the shelf will be a snap.

Some people compare laboratory chemistry with cooking in the kitchen. Sample preparation, dilution, and mixing are all done with measured care to produce a final product with specific characteristics and qualities. In other words, it might be a masterpiece or a mess, depending on how you follow the directions.

Practice problems for these concepts can be found at – Concentration and Molarity Practice Test

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