Calculating initial concentration

[Allina3337]

Homework Statement

calculate the initial concentration of HOAc and BCG in the HOAc stock solution. DOnt account for dissociation only dilution.

Given Info: Add 5 mL of 3.9 x 10^-4 M BCG and 25 mL of 1.000 M acetic acid into a flask. Dilute to a volume of 100 mL. This is the stock solution.

Relevant Equations

m1v1=m2v2

(1)(25)=(x)(5) and x=5 (initial concentration of HOA)
(x)(5)=(.200)(5) and x= 0.2 (initial concentration of BCG)

 

[Borek]

And? You clearly wrote what the problem is, but then you just wrote two random equations not even trying to explain what you are doing.

 

[Mike S.]

You can reduce your confusion by writing things a certain way and allowing your units to guide you. Note by "units" I don't just mean "SI units"; I mean a complete description of what you are actually measuring.

• M is an endless font of confusion, a useless and unnecessary pretension. Write it as mol/L at every opportunity.
• The dilute solution and concentrated solution are two different things - a mL of one is not the same as a mL of the other. So write these as "mL conc" and "mL dil" to keep track of which mL you are working with. Feel free to make up and write out any other "unit adjectives" to keep track of things you need as you go on - a mole of O is not a mole of O2, for example.
• Note that mol/L dil and mol/L conc. are now trackable as two different types of units also.
• ALWAYS keep your units in your calculations. It's perfectly fine to do a computation with all the units specified and just blanks for the numbers - that can show you you will get the right answer before you calculate. But never have the numbers specified and not the units!
• When solving for a ratio of two units (conversion factor) it is probably less confusing to figure each out separately.

I prefer to start by figuring out the units I need, then show how to get those by conversion factors and given data:

NEED: ? mol HOAc/mL dil

? mol HOAc = ( ___ mol HOAc / ___ mL conc) * ( ___ mL conc)

? mL dil = ...

Note ( ___ mol HOAc / ___ mL conc) is a conversion factor. That means it is equal to one; whenever you have so many moles, you have so many mL. If needed, you can split it up into a conversion factor it is more obvious you were given, and a metric conversion factor:

( ___ mol HOAc / ___ mL conc) = ( ___ mol HOAc / ___ L conc) * (____ L conc / ____ mL conc)

Observe that any multiplication of two conversion factors into one is a deduction, however elementary, and duplicates something you would think in your mind or in a separate calculation. Writing this makes the top line a bit longer, but you don't have to work the L to mL conversion factor in your head if that is still giving you even a moment's pause. (It shouldn't be, but it doesn't hurt to be cautious and explicit if there's any risk of getting mixed up)

 

[epenguin]

I'm wondering what BCG might be.

 

[Borek]

Most likely https://en.wikipedia.org/wiki/Bromocresol_green