[Maciejczyk]

Hello,

Please can someone explain this task extacly to me?

I would be grateful.

What mass of NaOH (Mr = 40) is required to make 0.5L of a 0.2M solution?

[sjb]

Hello,

Please can someone explain this task exactly to me?

I would be grateful.

What mass of NaOH (Mr = 40) is required to make 0.5L of a 0.2M solution?

What are your thoughts? What, for instance does Mr=40 mean, or 0.5L, 0.2M? Please check the forum rules.

[Borek]

Please read forum rules.

You have to show your attempts at solving the question to receive help. This is a forum policy.

[Arkcon]

This is a cute question -- I got almost this exact one during my first job interview.  I couldn't solve it.    But I got the job anyway.   That Mr for NaOH, what are the units for that value?  Try to find it, even if the problem doesn't specify the units.  It might help you understand this problem.

[Maciejczyk]

Oh, I am sorry, my bad. Here is description:

to determine the number of moles (mol) present or the molar (M or mol/L, moldm-3) concentration, where m is the mass in grams, Mr is the relative molecular mass (the mass of 1 mol of a chemical) and V is the volume in litres (L or dm3). Don’t confuse m, M, Mr and mol they represent different things.

mol =  m/Mr
M = m/( Mr x V)
mol = M x V

[AWK]

M = m/( Mr x V)
Hence calculate m.

[Maciejczyk]

Thank you!


Is this good result?

M = m/( Mr x V)
M = 0.5/(40 x 0.2)
M = 0.5 / 8 = 0,0625

[sjb]

Thank you!


Is this good result?

M = m/( Mr x V)
M = 0.5/(40 x 0.2)
M = 0.5 / 8 = 0,0625

What are your units?

[Maciejczyk]

M = m/( Mr x V)

M = 0.5L / (40(?) x 0.2M)
M = 0.5L / 8(?) = 0,0625(?)


Any advises?

[Borek]

Assuming Mr is a molar mass, what are units of molar mass?

[Maciejczyk]

Mr is the relative molecular mass (the mass of 1 mol of a chemical)

[Borek]

Sigh, stop dodging the question.

[Arkcon]

Mr is the relative molecular mass (the mass of 1 mol of a chemical)

Awesome.  So, One mole of NaOH weighs 40 ... units?  And once you have that, and you know molarity is moles per liter, and you know the molarity you want, and the volume (given to you as some fraction of a liter), can you then solve it -- keeping the units in pace after each number so you can keep track of everything and we (and your instructor on an exam) can see where you're getting everything?

[Maciejczyk]

Please tell me if these are right calculations for this task.
http://s15.postimage.org/4dippo83f/calculation.jpg

[Arkcon]

You get the correct answer.  Do you understand the meaning behind the labels on each box?  If you know it well, you'll be better off come examination time.  Or you know, when you actually have to prepare a solution in the laboratory.