# Find the Molarity of a Solution Using Concentrated Ammonia (28% by Mass)

• antonisz
In summary, molarity is a unit of concentration that measures the number of moles of a solute dissolved in a liter of solution. It can be calculated by dividing the number of moles of the solute by the volume of the solution in liters. To convert between molarity and molality, you will need to know the density of the solution and use the equation M = m/(M x ρ). The units of molarity are moles per liter (mol/L) and they can be used to determine the concentration of a solution and how much solute is present in a given volume of the solution. To calculate the molarity of a solution using titration data, use the equation M1V1 = M2V
antonisz

## Homework Statement

Calculate the molarity of a solution made by adding 35.5 mL of concentrated ammonia (28.0 % by mass, density 0.880 g/mL) to some water in a volumetric flask, then adding water to the mark to make exactly 250 mL of solution. (It is important to add concentrated acid or base to water, rather than the other way, to minimize splashing and maximize safety.)
u

## Homework Equations

Molarity = mol / liter

Mass % = (grams of solute / grams of solution) * 100

Density = mass / volume

## The Attempt at a Solution

a) I used the ml of NH3 to find the number of grams of NH3 - 35.5 ml * 0.88 g/ml = 31.24 g

b) I then used that amount to find the number of mole - (31.24 g / 17.03) = 1.8344 mol of NH3

This is where I am stuck. I used the % mass formula to find that the total grams of solution is 111.57 (0.28 = 31.24 / x and cross multiply), but I have no idea where to go from here.

I know that if I compute the molarity using (1.8344 mol / .250 L) I'll get the moles of NH3 at concentration 100%, would I need to multiply the number of moles by 0.28 to get the 28% concentration?

*Edit* Yes, it turns out that that would make sense, too much chemistry for the last 2 hours! I haven't taken Gen Chem in a couple years and getting thrown into Analytical, I should have reviewed some of these concepts. [/B]

antonisz said:

## Homework Statement

Calculate the molarity of a solution made by adding 35.5 mL of concentrated ammonia (28.0 % by mass, density 0.880 g/mL) to some water in a volumetric flask, then adding water to the mark to make exactly 250 mL of solution. (It is important to add concentrated acid or base to water, rather than the other way, to minimize splashing and maximize safety.)
u

## Homework Equations

Molarity = mol / liter

Mass % = (grams of solute / grams of solution) * 100

Density = mass / volume

## The Attempt at a Solution

a) I used the ml of NH3 to find the number of grams of NH3 - 35.5 ml * 0.88 g/ml = 31.24 g

b) I then used that amount to find the number of mole - (31.24 g / 17.03) = 1.8344 mol of NH3

This is where I am stuck. I used the % mass formula to find that the total grams of solution is 111.57 (0.28 = 31.24 / x and cross multiply), but I have no idea where to go from here.

I know that if I compute the molarity using (1.8344 mol / .250 L) I'll get the moles of NH3 at concentration 100%, would I need to multiply the number of moles by 0.28 to get the 28% concentration?

*Edit* Yes, it turns out that that would make sense, too much chemistry for the last 2 hours! I haven't taken Gen Chem in a couple years and getting thrown into Analytical, I should have reviewed some of these concepts. [/B]

In step (a), you have found the mass of ammonia solution . The mass of ammonia molecules in this solution is 28% of this. Remember that ammonia is a gas, the stuff you are working with is ammonia gas dissolved in water. Taking 28% of 31.24 g will tell you how many grams of ammonia are actually present in the solution. Continue on as you have done to calculate the final concentration.

antonisz said:

## Homework Statement

Calculate the molarity of a solution made by adding 35.5 mL of concentrated ammonia (28.0 % by mass, density 0.880 g/mL) to some water in a volumetric flask, then adding water to the mark to make exactly 250 mL of solution. (It is important to add concentrated acid or base to water, rather than the other way, to minimize splashing and maximize safety.)
u

## Homework Equations

Molarity = mol / liter

Mass % = (grams of solute / grams of solution) * 100

Density = mass / volume

## The Attempt at a Solution

a) I used the ml of NH3 to find the number of grams of NH3 - 35.5 ml * 0.88 g/ml = 31.24 g

b) I then used that amount to find the number of mole - (31.24 g / 17.03) = 1.8344 mol of NH3

This is where I am stuck. I used the % mass formula to find that the total grams of solution is 111.57 (0.28 = 31.24 / x and cross multiply), but I have no idea where to go from here.

I know that if I compute the molarity using (1.8344 mol / .250 L) I'll get the moles of NH3 at concentration 100%, would I need to multiply the number of moles by 0.28 to get the 28% concentration?

*Edit* Yes, it turns out that that would make sense, too much chemistry for the last 2 hours! I haven't taken Gen Chem in a couple years and getting thrown into Analytical, I should have reviewed some of these concepts. [/B]
I think you are making a mistake while calculating the mass of ammonia. The solution contains 28% by mass of ammonia. 35.5 x .88 will give you the mass of the solution, not NH3. Multiply that further by 0.28 to get the grams of ammonia. Then retry solving the problem.

siddharth23 said:
I think you are making a mistake while calculating the mass of ammonia. The solution contains 28% by mass of ammonia. 35.5 x .88 will give you the mass of the solution, not NH3. Multiply that further by 0.28 to get the grams of ammonia. Then retry solving the problem.
Yes, thank you. I caught my mistake. Just some careless thinking on my part.

Thank you for the response.

antonisz said:
Yes, thank you. I caught my mistake. Just some careless thinking on my part.

Thank you for the response.

## 1. What is molarity and how is it calculated?

Molarity is a unit of concentration that measures the number of moles of a solute dissolved in a liter of solution. It is calculated by dividing the number of moles of the solute by the volume of the solution in liters.

## 2. How do I convert between molarity and molality?

To convert between molarity and molality, you will need to know the density of the solution. Molarity is calculated using the volume of the solution, while molality is calculated using the mass of the solvent. You can use the equation M = m/(M x ρ) to convert from molality to molarity, where M is the molar mass of the solute and ρ is the density of the solution.

## 3. What are the units of molarity and how do I interpret them?

The units of molarity are moles per liter (mol/L). This means that for every liter of solution, there are a certain number of moles of the solute. Molarity can be used to determine the concentration of a solution and how much solute is present in a given volume of the solution.

## 4. How do I calculate the molarity of a solution using titration data?

To calculate the molarity of a solution using titration data, you will need to know the volume of titrant used, the concentration of the titrant, and the volume of the solution being titrated. Use the equation M1V1 = M2V2, where M1 is the molarity of the titrant, V1 is the volume of the titrant used, M2 is the molarity of the solution being titrated, and V2 is the volume of the solution being titrated.

## 5. What are some common mistakes when solving molarity problems?

Some common mistakes when solving molarity problems include using incorrect units, not converting between units when necessary, and forgetting to account for the volume of the solute added to the solution. It is important to carefully read the problem and double check your calculations to avoid these mistakes.

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