TBR GC Chapter 1 P7Q46

[1syed89]

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Hey, I am completely lost in figuring out this question. Thanks

To achieve the same chloride ion concentration as 1.0
Grams NaCl(s) dissolved into 100 mL solution, how
nany grams of MgCl2(s) must be added to enough water to make 100 mL of solution?

A) 1.0 x 1/2 x 58.4/94.9g MgCl2(s)

B) 1.0 x 2x 58.4/94.9 g MgCl2(s)

C) 1.0 x 1/2 x 94.9/58.4 gMgCl2(s)

D) 1.0 x 2 x 94.9/58.4 g MgCl2(s)

 

[MedPR]

Is it D?

(1g NaCl) * (2 MgCl2/1 NaCl) * (95g MgCl2/molMgCl2) * (1mol NaCl/58g NaCl).

Everything but g MgCl2 cancels out.

 

[pm1]

Is it D?

(1g NaCl) * (2 MgCl2/1 NaCl) * (95g MgCl2/molMgCl2) * (1mol NaCl/58g NaCl).

Everything but g MgCl2 cancels out.

shouldn't it be C because 1 mol of MgCl2 gives 2 mols of Cl- (2 normal) and you want the same concentration as NaCl (which gives only 1 mol of Cl- / 1 normal) ?

 

[1syed89]

Yea the answer is C. I just don't understand seem to grasp how to get to the answer.

 

[pm1]

Yea the answer is C. I just don't understand seem to grasp how to get to the answer.

okay, I'll try to explain.

So first of all you want to find out how many chloride ions do you get from 1 gram of NaCl. Once you do that you figure out how many grams of MgCl2 do you need to obtain the same amount of chloride ions.

(1) figuring out how many chloride ions from 1 gram of NaCl (MW = 58g)
1 g of NaCl (1mol NaCl / 58 g) (note that the relationship is one to one, you get one mole of Cl- per NaCl)

(2) lets say that we figured out it was X mols of Cl-, now we want to go from mols to grams
X mols (94.9g MgCl2/mol MgCl2) = this would give us the grams of MgCl2.

However, because the ratio of Cl- and MgCl2 is NOT one to one we would have to divide or multiply by the ratio. We know that MgCl2 gives us 2Cl- thus we just need half of the grams of MgCl2.

All together: we get C. Does that help?