Question #e73b2

2 Answers
Jun 15, 2015

Answer:

You need 79 mL of the stock solution.

Explanation:

The first thing you need to do is figure out exactly how much nitric acid your stock solution contains. To make the calculations easier, you can assume the you have a 1.00-L sample of the stock solution.

Use the solution's density to determine what the mass of the sample is

#1.00cancel("L") * (1000cancel("mL"))/(1cancel("L")) * "1.41 g"/(1cancel("mL")) = "1410 g"#

You know that this solution is 70.3% concentration by mass nitric acidm which means that every 100 g of solution contain 70.3 g of nitric acid. This means that you have

#1410cancel("g") * ("70.3 g"HNO_3)/(100cancel("g")) = "991.23 g"# #HNO_3#

Use nitric acid's molar mass to see how many moles would that many grams contain

#991.23cancel("g") * ("1 mole"HNO_3)/(63.01cancel("g")) = "15.73 moles"# #HNO_3#

This means that the 1.00-L sample will have a molarity of

#C = n/V = "15.73 moles"/"1.00 L" = "15.73 M"#

Now all you have to do is perform a simple dilution calculation

#C_1V_1 = C_2V_2#, where

#C_1#, #V_1# - the molarity and volume of the stock solution sample;
#C_2#, #V_2# - the molarity and volume of the target solution.

Plug your values into the above equation and solve for #V_1#

#V_1 = C_2/C_1 * V_2#

#V_1 = (0.500cancel("M"))/(15.73cancel("M")) * "2.5 L" = "0.07947 L"#

Rounded to two sig figs, the number of sig figs you gave for the volume of the target solution, and expressed in mL, the answer will be

#V_1 = 0.079cancel("L") * "1000 mL"/(1cancel("L")) = color(green)("79 mL")#

Jun 15, 2015

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 #HNO_3# solution.

#c=n/v#

So:

#n=cv#

So the no. moles #HNO_3# required = #2.5xx0.5=1.25#

Now we need to find how many grams this is. We get this by multiplying by the weight of 1 mole of #HNO_3# which is equal to its #M_r# in grams.

#M_r[HNO_3]=63.01#

So mass required = #1.25xx63.01=78.76"g"#

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

This means that 100g of this concentrated solution contains 70.30g of #HNO_3#.

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

There are:

#70.30"g"# # HNO_3# in #100"g"# of concentrated solution.

#1"g"# is present in #(100)/(70.30)"g"# of concentrated solution.

So:

#78.76"g"# is present in #(100)/(70.30)xx78.76=112.03"g"# of concentrated solution.

The density of the solution = 1.41g/ml

density = mass/volume

So volume = mass/density

So the volume required = #(112.03)/(1.41)=79.45"ml"#