Struggling with Molarity promblems!! Please Help !!??

Question

Please, can anybody tell me how to figure out the problems that my teacher gave to my class to review for an AP chemistry Test shes giving us a in a couple of days and I have no clue how to solve these problems.

The following problems are:

1) Calculate the molarity of C2H5OH in a water solution that is prepared by mixing 50 ml of C2H5OH with 100 ml of Hs0 at 20 degrees Celsius. The density of the C2H5OH is 0.789 g/ml at 20 degrees Celsius.

2)What is the molarity of a solution of 50 g of propanol (CH3CH2CH2OH) in 152 ml water, if the density of water is 1.0 g/ml.

3). A solution containing 296.6g of Mg(NO3)2 per liter has a density of 1.114 g/ml. The molarity of the solution is what?

Answer 1

Okay, let's walk through it...

You start off with 50 ml of ethanol (C2H5OH) in water. How many grams of ethanol do you have if the density is 0.789 g/ml?

Answer: 50 ml * 0.789 g/ml = 39.45 grams of ethanol.

So how many mols of ethanol do you have? For this you need to know the molecular weight of ethanol.

You can see that the formula has 2 carbon, 6 hydrogen and one oxygen. If you look up the atomic weights for these elements, they are 1.008 for hydrogen, 16.000 for oxygen, and 12.011 for carbon. The molecular weight is therefore:

(6 * 1.008) + (16.000) + (2 * 12.011) = 46.07.
This is the molecular weight of ethanol.

How many mols do you have? Now it's easy... you have 39.45 grams, divided by 46.07 grams per mol, equals 0.8563 mols.

Now how many total liters do you have? (Here the question itself is flawed, since you need to assume that the 50 ml of alcohol in 100 ml of water gives you 150 ml of solution. This is a dangerous assumption since it is not the case for all pairs of substances). Anyway, let's assume that the total volume is now 150 ml, or 0.150 liters. The solution strength is 0.8563 mols per 0.150 liters, or 5.71 mols per liter.

To summarize the steps:

(a) Figure out how many mols of ethanol you have. To do this you need to convery volume to grams, then grams to mols.
(b) Figure out your total volume of solution. (Molarity is expressed as mols per liter of solution, not mols per liter of water. You have only 100 ml of water, but you have 150 ml of solution once you add the ethanol).
(c) Divide the number of mols by the number of liters to get molarity.

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Question #2 is so flawed that it cannot be answered. (Sorry about that, but unfortunately standardized exams are so poorly written that they are frequently unanswerable). Let's see how far we can go:

You are given the number of grams of propanol, so all you need is the molecular weight of propanol in order to know the number of mols. The formula is C3H8O, which has a molecular weight of (3*12.011) + (8*1.007) + 16.000, or 60.097.

The number of mols is equal to 50 grams / 60.097 grams/mol, or 0.832 mols of propanol.

Now how many liters of solution do you have? Here you reach a roadblock, since you know how many liters of water you have, but you have no way of calculating liters of (water + propanol) mixture. Remember, you need mols of propanol per liter of solution, not mols per liter of water. I really am sorry about this... it's confusing enough to learn this stuff without the questions themselves being dumb-assed.

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For question #3, you are told that a liter of solution has a density of 1.114 g/ml. This is completely unnecessary information - you don't need it to complete the question and it's there just to fool you. All the info you need is in the first statement - the concentration is 296.6 grams per liter.

How many grams are there in a mol? There is one magnesium (24.305 grams/mol) plus two nitrogens (14.007 grams/mol * 2) plus six oxygens (16.000 grams/mol * 6) or 148.319 grams/mol.

The molarity of the solution is 296.6 g/l / 148.3 grams per mol = 2.000 mol/liter.

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It's a really, really good idea to keep track of your units, not only the numbers themselves... helps you avoid mistakes. For example, in #3 the units are as follows:

(g/liter) divided by (g/mol)

This simplifies to (mol/liter). It is ALWAYS worth doing this check when you calculate things. If your units come out to liters/mol, for example, it immediately tells you that you did something backwards in your calculations.

Answer 2

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Answer 3

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