Tbr chemistry passage 8

[Temper888]

SALT
MASS
VOLUME WHEN HEATED
VOLUME W/H20 added
MX
7.0g
26.2 mL (@ 31.6 C)
28.9L mL (@25.0 C)
MY
9.0g
25.3 mL(@ 27.4 C)
26.1 mL (@ 25.0 C)
MZ
4.0g
25.7 mL (@30.3 C)
28.7 mL (@ 25.0 C)

MX = 100 g/mole , MY=120 g/mole , and MZ 150 g/mole


1) Which of the following solutions has the GREATEST molarity?
A. Saturated MX at 31.6 C
B. Saturated MX at 25.0 C
C.Saturated MY at 27.4 C
D. Saturated MY at 25.0 C


ANSWER EXPLANATION: Because solubility of salt increases with temperature answer choices B and D are eliminated. It can also be read from the third column in table that more MX and MY dissociate at a temperature greater than 25 C than what dissociates at 25 C. Answer is C from calculations.


2) As temperature increases, what happens to the various concentration measurements of a solution, assuming no evaporation and no addition of solute?
A. The density and molarity both increase.
B.The density increases, while the molarity decreases.
C. The density decreases, while the molarity increases.
D.The density and molarity both decrease.


ANSWER EXPLANATION:As temperature rises, the volume of the solution increases according to TABLE 1. Liquids, as a general rule, expand with increasing temperature. As the volume increases, both the molarity and the density of a solution decrease. Answer is D.


My questions are: 1) Volume decreases when heat is added, but in answer explanation 2 it says otherwise?How?

2) The two answer explanations are contradicting? How can volume decrease in one answer and increase in other when temperature is increased?
3) Do Molarity and Density both decrease when temperature is increased because volume is increased in all scenarios?

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[Ssina]

So as a general rule, as you mentioned, when you head a liquid, it's volume increase... and since density is m/V and molarity is moles/V... when you increase the volume then you decrease the density and molarity (since neither the mass or #moles changes as we are not adding anything)

in the first question, they are making a saturated solution by mixing e.g. 7g of MX into a solution of water... if they use a water that is at 31.6C they need to use less water, only 26 ml as oppose to 28ml if the water is at 25C to make the solution. this is because the solubility is greater at higher temperature. so here the volume is not changing because of the temperature, instead they are using different amounts to being with.

 

[MedPR]

My questions are: 1) Volume decreases when heat is added, but in answer explanation 2 it says otherwise?How?

2) The two answer explanations are contradicting? How can volume decrease in one answer and increase in other when temperature is increased?
3) Do Molarity and Density both decrease when temperature is increased because volume is increased in all scenarios?

1. Melt an ice cube. Which has more volume? The solid water or the liquid water?
2. Can you elaborate on where the explanations are contradictory? I don't see it that way.
3. Molarity is moles/volume, Density is mass/volume. If you increase the volume without changing anything else, then yes molarity and density will both decrease. However, if you increase temperature, salts increase in solubility, so molarity will go up if you don't add any liquid.

 

[JD7]

9.0g
25.3 mL(@ 27.4 C)
26.1 mL (@ 25.0 C)

Even if they increased the temperature you can see the volume of water utilized decreased.

 

[MedPR]

9.0g
25.3 mL(@ 27.4 C)
26.1 mL (@ 25.0 C)

Even if they increased the temperature you can see the volume of water utilized decreased.

Yes, because molar solubility increases as temperature increases, so you don't need as much solvent to dissociate the same amount of solute.

The quoted data is not indicative of volume being lower when temperature is lower. It is indicative of solubility being higher when temperature is higher.

Again, if you melt a solid, its volume has now increased thus decreasing its molarity and density assuming no solute was added. This is something you know (should know) independently of the data presented. The fact that solubility increases with temperature is also something you know (should know) independently of the data, but the data also tells you that.

 

[drechie]

just did this passage and got the #2 question wrong. can someone re-explain this or link it to a video / image where i can learn. i dont understand how as you increase the temperature, volume would increase.

 

[DrknoSDN]

just did this passage and got the #2 question wrong. can someone re-explain this or link it to a video / image where i can learn. i dont understand how as you increase the temperature, volume would increase.

Fluids expand very slightly when heated. It might be easiest to google "density with increasing temperature". It's more common to observe density changes than volume changes.

Of significant importance. If you are trying to prepare a 1 molar solution of anything, the order must be to add 1 mol solute, then fill water line to exactly 1 liter. If you fill flask to 1 liter than add solute the total volume will be slightly more than 1 L.
Question 2 is similar in that if you increase temperature, volume of most anything will expand so you will have the same number of moles, but larger volume (lower density) and decreased mol/L.

 

[drechie]

thank you so much for that ^^^.

For those who may be puzzled by this:

• Heating a substance causes molecules to speed up and spread slightly further apart, occupying a larger volume that results in a decrease in density.
• Cooling a substance causes molecules to slow down and get slightly closer together, occupying a smaller volume that results in an increase in density.
• http://www.middleschoolchemistry.com/lessonplans/chapter3/lesson6

 

[Premedstudent500]

I think you are assuming that it is talking about the solution in the passage, but #2 is kinda like a discrete, and it is not really referring to the solutions in the passage. The solutions in the passage are saturated with a precipitate and so in experiment 1 when we increase the temperature, more of the salt is able to dissolve because there is some precipitate that hasn't dissolved. increase in dissolved moles= molarity increases.

But the question stem doesn't say its saturated, so we assume like most solutions we have worked with in the past that it is unsaturated. Increasing the temperature to a solution that is unsaturated doesn't increase the dissolved moles... because all of it is already dissolved in the solution. So in an unsaturated solution, increasing the temperature really only changes the volume of the liquid thereby changing the density and the molarity.

 

[chemtopper]

[QUOTE/]My questions are: 1) Volume decreases when heat is added, but in answer explanation 2 it says otherwise?How?
2) The two answer explanations are contradicting? How can volume decrease in one answer and increase in other when temperature is increased?
3) Do Molarity and Density both decrease when temperature is increased because volume is increased in all scenarios?[/QUOTE]
two questions are based on two different situations.
First question -we are comparing three different solutions(saturated MX ,MZand MY) at two different temperatures.From the data it is clear that we can dissolve more MX,MY and MZ at higher temperature in less volume.When you heat a saturated solution ,solubility increases and you need less volume to dissolve same amount of the solute.Here safely I can rule out options at low temperature as same amount of the solute is dissolved in more volume(this conclusion is based on same e.g MX solute solutions at different temperatures) .Molarity is moles per liter and more volume means less molarity for the same no of moles of the solute.
Now at high temp we have three solutions MX ,MYand MZ.we will calculate molarity of each solution at high temperature.We know moles of each solute and volume also:

moles of MX=7/100
volume = 26.2mL=0.0262L
molarity= 0.07/0.0262=2.67M
moles of MY=9/120
volume = 25.3mL=0.0253L
molarity=2.96M
moles of MZ=4/150
volume = 25.7mL=0.0257L
molarity=1.03M
Now you can pick up the right option.
IN THE SECOND QUESTIONS :
They are talking about the same solution and not comparing different solutions.
MX saturated solution becomes unsaturated on raising the temperature because on heating there is slight increase in volume of the solution.As volume increases in a solution with no change in mass/moles(quantity of the solute) ,this will decrease the density and molarity.Density and molarity are inversely related with volume.