gc qvault question

[Troyvdg]

Question 1

When tested, an unknown solution fails to conduct electricity. Which of the following combinations of solute and solvent could be the unknown?

A. NaCl dissolved in water
B. table sugar dissolved in water
C. Glycerol dissolved in water
D. steric acid dissolved in butanol
E. napthalene dissolved in n-hexane *answer

I understand why answer choice E would not conduct electricity, and why answer A would...but I'm having trouble understanding the rest....table sugar in water? why would that conduct electricity, sugar is a non electrolyte.


Question 2

The concentration of a pollutant in a sample of rainwater is found to be 3ppm. How many grams of the compound will be present in 2L of water? (Assume that the density of the rainwater is equivalent to the density of pure water).

Qvault says the answer is 6 x 10^-6
I keep getting 6 x 10^-3



Any help is appreciated, thanks!

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[ReaverOpS]

It's probably because the rest (solute and/or solvent alike) can form ions, which allows for the conduction of electricity.

 

[Troyvdg]

It's probably because the rest (solute and/or solvent alike) can form ions, which allows for the conduction of electricity.

but don't we assume pure water in these types of questions? which would not conduct electricity

 

[ReaverOpS]

but don't we assume pure water in these types of questions? which would not conduct electricity

We do assume pure water, and pure water doesn't conduct electricity, but this question is asking whether an added solute combined with the solvent water will conduct electricity, and this new entity will conduct electricity. Choice A definitely will conduct electricity since NaCl can form electrolytes, which will conduct electricity in water.

For Choice B, keep in mind that table sugar dissolves in water because when a sucrose molecule breaks from the sugar crystal, it is immediately surrounded by water molecules. The sucrose has hydroxyl groups that have a slight negative charge. The positive charge of the oxygen found in the water molecule binds with the sugar. As the hydration shell forms around the sucrose molecule, the molecule is shielded from other sugar molecules so the sugar crystal does not reform. This, in turn, can somewhat conduct electricity, even if it does so weakly. A similar reasoning can be applied for Choice C. The three hydroxyl groups in glycerol will make it easily soluble in water via the formation of hydrogen bonding with water.

In Choice D, both the solute and solvent are polar.

Hope this helps.

 

[Troyvdg]

We do assume pure water, and pure water doesn't conduct electricity, but this question is asking whether an added solute combined with the solvent water will conduct electricity, and this new entity will conduct electricity. Choice A definitely will conduct electricity since NaCl can form electrolytes, which will conduct electricity in water.

For Choice B, keep in mind that table sugar dissolves in water because when a sucrose molecule breaks from the sugar crystal, it is immediately surrounded by water molecules. The sucrose has hydroxyl groups that have a slight negative charge. The positive charge of the oxygen found in the water molecule binds with the sugar. As the hydration shell forms around the sucrose molecule, the molecule is shielded from other sugar molecules so the sugar crystal does not reform. This, in turn, can somewhat conduct electricity, even if it does so weakly. A similar reasoning can be applied for Choice C. The three hydroxyl groups in glycerol will make it easily soluble in water via the formation of hydrogen bonding with water.

In Choice D, both the solute and solvent are polar.

Hope this helps.

But when sugar is dissolved in water, no new ions are created. Therefore it wouldn't be able to conduct charge, because the ion concentration would be the same as pure water, just with sucrose in the solution. Right?

 

[ReaverOpS]

Technically, a solution of sugar molecules (particles without charge) would not be ionizable in water but since there is a slight polar interaction between the sugar and water molecules, this is not as good of a choice as Choice E, where both sugar and solvent are completely nonpolar, thus preventing any electrical conduction.

Personally, I think the question is a bit poorly written; I understand your confusion

 

[fit2]

I didn't like the question either. I remember from lab that water and sugar did not produce electricity. Oh well...it probably won't show up on the exam with the same wording. It's good to keep in mind that a miscible non polar solution can't conduce electricity.