Question

A student dissolved 3.00 g of Co(NO3)2 in enough water to make 100. mL of stock solution. He took 4.00 mL of the stock solution and then diluted it with water to give 275. mL of a final solution. How many grams of NO3- ion are in the final solution?

Answer 1

Answer:

0.081g

Explanation:

first step:calculate the molar conc of the stock solution

molar conc=mass/molar mass ×1000/vol(ml)

molar mass of Co(N03)2=182.943

molar conc=3g/182.943 ×1000/100ml

=0.163M

To calculate the conc of the resulting solution we apply the dilution principle

n=cv

C1V1=C2V2

C1=conc of stock solution=0.163M

V1=4mL

V2=275ml(final volume)

by making 'C2" the subject C2=0.163×4/275

=0.0024M

by applying the formular for calculating molar conc of a solution,we can calculate the mass of CO(NO3)2 in the reulting solution

molar conc=mass/molar mass ×1000(L)/vol(mL)

mass=0.0024×182.943×275/1000

=0.1193g

Mass of CO(NO3)2  in the resulting solution=0.1199g

since CO+(NO3)2=CO(NO3)2

Percentage by mass of (NO3)2 in CO(NO3)2=(NO3)2/CO(NO3)2

=124/182.943×100

=67.8%

it means 67.8% of 0.1193g is the mass of (NO3)2=0.678×0.1199

=0.081g