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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: alecia2011 on May 17, 2011, 10:19:50 PM

Title: Desperate --> g NaOH dissolved in H2O to give 500cm3 solution.. calculate molary
Post by: alecia2011 on May 17, 2011, 10:19:50 PM

A solution is prepared from 12 g NaOH dissolved in enough water to give 500 cm³ of solution; calculate the molarity of the solution.

I have tried so many ways and just get completely stuck.  I don't know where to go anymore or even how to begin this.  pleasee help me solve this.

I've tried:

1.) 12g NaOH * 1cm3/2.13g --> 5.634cm3 NaOH
        5.634cm3 NaOH/500cm3 solution --> 1.13% weight
             .0113 = 12g NaOH/X --> X=1065g solution
  so theres 12g NaOH and 1053g H2O...


I guess I just need help setting up the dang thing, or the steps on how to solve it.

Title: Re: Desperate.
Post by: enahs on May 17, 2011, 10:41:15 PM

What is the definition of molarity?

Title: Re: Desperate.
Post by: alecia2011 on May 17, 2011, 10:46:19 PM

Quote from: enahs on May 17, 2011, 10:41:15 PM

What is the definition of molarity?

Molarity = mol/L

mol = mass/MW

density of NaOH as 2.13g/cm3

i know cm3 can be interchanged with mL

Title: Re: Desperate --> g NaOH dissolved in H2O to give 500cm3 solution.. calculate molary
Post by: enahs on May 18, 2011, 12:44:19 AM

So, you know everything.

Density is not needed.

You have 12 g of NaOH, so you have how many mols?
How many liters are in 500 cm3 (as you said, 500mL)?


Again, as you said mols/L. Plug in your mols and your L.

Title: Re: Desperate --> g NaOH dissolved in H2O to give 500cm3 solution.. calculate molary
Post by: AWK on May 18, 2011, 01:48:41 AM

Quote

Molarity = mol/L

Just convert mass to moles and put number in numerator, then convert mL to Liters and put this number in denominator - finally divide these numbers

Title: Re: Desperate --> g NaOH dissolved in H2O to give 500cm3 solution.. calculate molary
Post by: ibrahim_kaream on November 14, 2011, 03:40:44 PM

Mt. Wt. of NaOH = 40

you dissolved 12 gm NaOH to 500 ml water (0.5 L)

So,
12 gm ------- 0.5 L
x   gm-------  1 L

x=12/0.5 = 24 gm

So, You prepared a soln. of 24 gm/L

1 M NaOH ----- 40 gm/L
x M NaOH ----- 24 gm/L

X=24/40= 0.6 M

Conclosion: You prepared 0.6 M NaOH by dissolving 12 gm in 500 ml water.

NB: ml = cm3

Title: Re: Desperate --> g NaOH dissolved in H2O to give 500cm3 solution.. calculate molary
Post by: Borek on November 14, 2011, 06:40:38 PM

First: giving final answers is against forum rules (http://www.chemicalforums.com/index.php?topic=33740.0).

Second: this thread is 6 months old.

Title: Re: Desperate --> g NaOH dissolved in H2O to give 500cm3 solution.. calculate molary
Post by: ibrahim_kaream on November 17, 2011, 03:24:31 PM

to calculate molarity all you need is just calculate the prepared concentration in the form of gm/Liter, then divide the result by the molecular weight of NaOH.

12 gm/500 cm3=12 gm/500 ml=12 gm/ 0.5 L=24 gm/ L

M=24/40(NaOH molecular weight)=0.6 M