BR GCHEM Passage 7 Question 46

[MCATMadness]

Can someone explain the unit factor method for this question?

I understand conceptually why we multiply by 1/2 but I do not understand how to get 1/2 using the unit factor method...I would have said there are 2 moles of Cl in MgCL2 and 1 mole of Cl in NaCl... and putting that into the unit factor method would have resulted in multiplying by 2.

does anyone understand this?

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[BerkReviewTeach]

Can someone explain the unit factor method for this question?

I understand conceptually why we multiply by 1/2 but I do not understand how to get 1/2 using the unit factor method...I would have said there are 2 moles of Cl in MgCL2 and 1 mole of Cl in NaCl... and putting that into the unit factor method would have resulted in multiplying by 2.

does anyone understand this?

You are trying to get the same number of chloride ions into 100 mL solution whether you use 1.0 g NaCl or some undetermined amount of MgCl2. So the question at its core is asking how many grams of MgCl2 will give you the same number of chloride ions as 1g NaCl?

1g NaCl x 1 mole/58.4 g x 1 Cl/1NaCl x 1MgCl2/ 2 Cl x 94.9 g/mole

Conceptually, if you think that MgCl2 has two Cls instead of 1, then it makes sense you need only 1/2 as much to get the Cls you need.