help
given ammonia from stock 25%v/v
need to get 100 cm^3 ammonia that is 2M/L
density of stock solution is 0.9 g/cm^3
what amount of stock solution do i need?

Question

anonymous · Answer

I can help you with this (dwaddling in the airport for the next hour) but I'm gonna need some help understand what "v/v" means??

jfraser · Answer

v/v is a "volume/volume" percentage. Out of every 100mL of solution, 25mL is ammonia, the remainder is water. You need to find out how many mL of the concentrated stock you need to dilute in order to make 100mL of the more dilute solution.

If the final volume and concentration needs to be 0.100L of 2.0mol/L, then you need
$$moles = (conc)(volume)$$$$moles = (2.0M)(0.100L) = 0.2mol NH{_3}$$
The volume of concentrated ammonia you take needs to have 0.2moles of solute. The mass of ammonia must be
$$mass = (moles)(molarmass)$$$$mass = (0.2mol)(17g/mol) = 3.4g NH{_3}$$
Use the density of the solution to convert the mass into volume
$$volume = mass/density$$$$volume = 3.4g/(0.9g/cm{^3}) =3.8cm{^3}$$
So the volume of pure ammonia needed to make the solution is 3.8mL, but only 25% of the stock is ammonia. That 3.8mL is going to be 25% of what you take, so
$$totalvolume = 3.8mL/(0.25) = 15.1mL$$
where 0.25 is the percentage of the solution.
Taking 15.1mL of the stock ammonia and diluting to 100.mL will get you a final concentration of 2.0M

anonymous · Answer

thank you so much lifesaver