# Question #5be33

##### 2 Answers

**!! LONG ANSER !!**

**SIDE NOTE** *This in not the correct way to solve this problem, because it assumes that all the volume of the salt will add to that of the calcium chloride solution; this is not correct, like Alfredo showed in his answer and in the later comments.*

*However, I will leave the answer as it is because I want students to be able to see the mistake I made in solving this problem.*

So, here's how you'd go about solving this problem.

You know that you are dealing with a **0.250-M** solution of calcium chloride in an *initial* volume of

Notice that you have a **twice as many** moles of

At the same time, the

Use the solution's molarity to determine how many moles of each you get

This means that you get

and

Now you add the **2.25 g** of sodium chloride. Because the volumes are additive, you must determine what volume that much

Once you add it to the solution, the number of moles of sodium cations and of chloride ions will be

and

The *total* volume of the solution will be

*It is safe to assume, given the lack of information provided, that the total volume of the solution will not be changed; it is beyond the scope of the given data to show that it does actually change, but by less than 1.04 mL.*

The *total* number of chloride ions will be

Now just solve for each ion's concentration by using *the total volume*

This kind of problem is solved by

1. determining the *quantities* of each kind of solute and ion, then

2. summing the quantities of ions of the same species, and finally

3. dividing each ion amount by the *total* volume, to determine the corresponding *concentration*.

This is best accomplished by converting firstly the *quantities* of substance in moles.

The problem is badly formulated, because it refers to an *additive* mixing of volumes of liquid solutions, whereas the mass of 2.25 g of NaCl is supposedly added as a solid to the 50 cm³ of calcium chloride solution.

Anyway I will try to solve the problem by interpreting the "additivity" criterion to mean that the volume of calcium chloride solution will remain approximately 50 cm³ after the addition of 2.25 grams of NaCl.

**1. Calculate the amount of each substance in moles**

Molar mass of NaCl =

moles of 2.25 g

Moles of

**2. Calculate the amount of each ion in moles** *(the two salts are strong electrolytes i.e. completely dissociated in ions)*

Therefore,

**Calculate the (molar) concentration of each ion species**