Problem 119P

Nitric acid is usually purchased in concentrated form with a 70.3% HNO3 concentration by mass and a density of 1.41 g/mL. How much of the concentrated stock solution in milliliters should you use to make 2.5 L of 0.500 M HNO3?

#### Answer:

You should use 79.45 ml of the stock solution and make it up to a volume of 2.5L

#### Explanation:

You need 2.5L of 0.5M HNO3 solution.

C = nV

So:

n = CV

So the no. moles HNO3 required = 2.5×0.5=1.25

Now we need to find how many grams this is. We get this by multiplying by the weight of 1 mole of HNO3 which is equal to its Mr (mass required) in grams.

Mr[HNO3]= 63.01

So mass required = 1.25×63.01=78.76g

We know that the concentrated form is 70.30% by mass.

This means that 100g of this concentrated solution contains 70.30g of HNO3.

So now we need to work out what volume of concentrated solution will give us the mass we need:

There are:

70.30g HNO3 in 100 g of concentrated solution.

1g is present in 100/70.30g of concentrated solution.

So:

78.76g is present in 100/70.30×78.76=112.03g of concentrated solution.

The density of the solution = 1.41g/ml

density = mass/volume

So volume = mass/density

So the volume required = =79.45ml