Question about Density - Can anyone explain this?

[ilovemcat]

I was doing a BR Gen. Chem Passage (Page 209, Passage VIII) if anyone has TBR General Chemistry Book. It's hard to explain the question without extrapolating passage information so if anyone who has the book can provide input, it'd be greatly appreciated. The Question number is 55.


I'll try my best to re-word the question:

Say I have a 28.9 mL solution composed of 7 grams of a salt (MX) at 25 degrees Celsius. I also know that at 25 degrees Celcius, water has a density of 1 g/mL.

The solution is explaining that "At 25 C there are 7 grams of MX and 25 grams of H20 in 28.9 mL of solution." I don't understand how they knew there was 25 grams of water. If anything, I would assume there was 28.1g - 7g = 21.1 g of water ..not 25 grams. I don't understand the whole relationship with density. It's driving me up the wall. Can anyone explain this?

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[MCAT guy]

I was doing a BR Gen. Chem Passage (Page 209, Passage VIII) if anyone has TBR General Chemistry Book. It's hard to explain the question without extrapolating passage information so if anyone who has the book can provide input, it'd be greatly appreciated. The Question number is 55.


I'll try my best to re-word the question:

Say I have a 28.9 mL solution composed of 7 grams of a salt (MX) at 25 degrees Celsius. I also know that at 25 degrees Celcius, water has a density of 1 g/mL.

The solution is explaining that "At 25 C there are 7 grams of MX and 25 grams of H20 in 28.9 mL of solution." I don't understand how they knew there was 25 grams of water. If anything, I would assume there was 28.1g - 7g = 21.1 g of water ..not 25 grams. I don't understand the whole relationship with density. It's driving me up the wall. Can anyone explain this?

This is simple actually. Water, 1 g = 1 mL; 25 mL = 25 g.

All they are saying is that if you start with 25 mL you have 25 grams, the addition of 7 grams of MX doesn't raise the volume to 32 mL, it only raises the volume to 28.9 mL at the current temperature.

You can't say 28.9 grams of anything, there was never any hint of 28 grams...

This is all good I guess, I don't see AAMC harping on this or getting this complicated (maybe 1 question of 52 possibly on an MCAT?)