MCAT C/P Question HELP!

[mariposas905]

How do you do this question - C/P Section Bank Q73? I feel like the AAMC explanations are terrible...it didn't help me understand how to solve this at all

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[BerkReviewTeach]

This is a unit conversion question. They give you molarity of the solution and a mass of the salt and ask for volume of solution. From the periodic table you can get molar mass. So this is a gram-to-mole-to-volume of solution conversion.

0.5 mg MgCl2 x 1 mole MgCl2/95 g MgCl2 x 1 L sol'n/0.01 moles MgCl2 = volume needed

Simplifying the math by rounding 95 to 100, we get 0.5 mg x 1/100 mole/g x 1/0.01 L/mole = 0.5 x .01 x 100 = 0.5 mL = 500 µL.

Set up the unit conversion and it becomes clearer.

 

[mariposas905]

Thank you for your reply @BerkReviewTeach ! Is C/P Section Bank Q57 also a unit conversion question? I didn't understand the way they solved it...