Question

5 gm naoh dissolve in 200 ml water

Answer 1

The way this question is written makes the answers given so far incorrect. The other answers are most likely what the question is looking for, but not technically correct. To correctly answer this question requires that you know the density of a 2.5% solution of NaOH. Why? Molarity is Moles Solute/Volume of solution. We are given volume of solvent, not of solution. The solution is 2.50% NaOH (5/(200*0.9982)*100%)=2.5047%~2.50%

The density of a 2.5% solution of NaOH is 1.0263 g/mL

The total mass of the solution is 204.6 g (5 g NaOH+ 200mL*0.9982 g/mL) so the total volume is 204.6 g/( 1.0263g/mL)=199.36mL=0.19936L

Now the Molarity can be calculated.

5 g/(39.997 g/mol)/0.19936L=0.62705M~0.627M

We can see that the difference between the molality calculation (which is what the other two answer gave and the molarity calculation only appears in the third significant digit. This difference becomes smaller for more dilute solutions and larger for more concentrated solutions.

tl;dr This question is poorly written and requires the student to make assumptions or look up additional data. Unambiguous phrasing of the problem would be something like:

5 g of NaOH is mixed with enough water to make a 200 mL solution. What is the molarity of the solution?

or

5 g of NaOH is mixed with 200 g of water. What is the molality of the solution?
86 Views · View Upvoters
Upvote · 1
Share

Rohan NagpalComment...
Promoted by Blinkist

A smarter you is only 15 minutes away. Try the Blinkist app.
Read or listen to the world's best nonfiction books in a matter of minutes and become a smarter you.
Get the App
James Emerson
James Emerson, attended graduate school for chemistry
Answered May 18
Molarity is moles per liter or mol/L.

One must find moles.

Find the number of moles of NaOH using the formula weight (molecular mass) for NaOH. (23 + 16 + 1 = 40 grams NaOH per mole of NaOH.

5 grams NaOH * (mole NaOH/40 g NaOH) = moles of NaOH = 1/8 or one-eighth of a mole or 0.125 mol NaOH. The gram units cancel and is better illustrated as a fraction, leaving moles in the numerator.

Now with the volume as 200 mL, one could just say 0.20 L.

O.125 mol/0.20 L = answer.
17 Views
Upvote
Share

Rohan NagpalComment...
Malcolm Sargeant
Malcolm Sargeant, Degree level applied chemistry + 20yr experience in corrosion prevention and water treatment
Answered May 18
5 grams in 200mL is 25 grams in a litre, the molecular mass of NaOH is 40 so the molarity is 24/40
26 Views
Upvote
Share

Rohan NagpalComment...
Antonio Daniels-Azerbaidzhan
Antonio Daniels-Azerbaidzhan, former Ind. Hygiensit-Safety Specialist-marine Chemist
Answered May 13
5g/40 =0.125 molNaOH

molarity = 0.125 mol/.200 L = 0.625 molarity

Answer 2

To correctly answer this question requires that you know the density of a 2.5% solution of NaOH. Why? Molarity is Moles Solute/Volume of solution. We are given volume of solvent, not of solution. The solution is 2.50% NaOH (5/(200*0.9982)*100%)=2.5047%~2.50%

The density of a 2.5% solution of NaOH is 1.0263 g/mL

The total mass of the solution is 204.6 g (5 g NaOH+ 200mL*0.9982 g/mL) so the total volume is 204.6 g/( 1.0263g/mL)=199.36mL=0.19936L

Now the Molarity can be calculated.

5 g/(39.997 g/mol)/0.19936L=0.62705M~0.627M

We can see that the difference between the molality calculation (which is what the other two answer gave and the molarity calculation only appears in the third significant digit. This difference becomes smaller for more dilute solutions and larger for more concentrated solutions.

tl;dr This question is poorly written and requires the student to make assumptions or look up additional data. Unambiguous phrasing of the problem would be something like:

5 g of NaOH is mixed with enough water to make a 200 mL solution. What is the molarity of the solution?