In Mole Calculations, many students lose marks in questions related to Volumetric Analysis, especially on those that requires them to understand the concepts of CONCENTRATIONS of solutions. Volumetric Analysis are always a highlight in our annual Mole Calculations Mastery Workshop.
Concentrations of a solution refers to the amount of solute dissolved in 1 dm3 of the solution.
Do note that:
Solution = Solute (minor component) + Solvent (major component)
Usually in basic chemistry such as O Levels, the solvent used is Water. As such, the solution formed is usually aqueous solutions whereby certain solid or liquid are dissolved in water.
Concentrations can be expressed in:
- Grams of solute per dm3 (unit will be g/dm3)
- Moles of solute per dm3 (unit will be mol/dm3)
The most important formulae student need to know in volumetric analysis are:
Conc. (mol/dm3) = Amt. of Solute (mol) / Vol. of Solution (dm3)
&
Conc. (g/dm3) = Amt. of Solute (g) / Vol. of Solution (dm3)
Let’s check out a question to see how we can use the formulae above.
Question:
60g of NaOH is dissolved in water and make up to the total volume of 500cm3. Calculate the concentration of solution in i) g/dm3 and ii) mol/dm3.
Answer:
i)
Mass of NaOH = 60g
Volume of solution in dm3 = 500/1000 = 0.5dm3
Concentration of solution in g/dm3 = 60 g /0.5 dm3 = 120 g/dm3
ii)
Mr, Relative molecular mass of NaOH = 23 + 16 + 1 = 40
Moles of NaOH = Mass / Mr = 60/40 = 1.5 mol
Concentration of solution in mol/dm3 = 1.5 mol / 0.5 dm3 = 3 mol/dm3
Isn’t it easy if you know the strategy of doing it? I would love to hear from you. Leave me a comment.
For those itching to try out a question on your own. Check out the Quick Check below with a little bit of twist.
Quick Check 1:
Given concentration of a NaOH solution is 1.5 mol/dm3. How many grams of NaOH solute are contained in 2.0 dm3 of solution?
PS: Try it out and leave your suggested answer (working even better) below! Love to hear from you.
uhhhh.
I hope i did it correctly, lol.
Moles of NaOH = 1.5mol/dm3 x 2.0dm3
= 3.0 moles
Therefore grams of NaOH = 3.0 x (16+1+23)
= 3.0 x 40
= 120g
Reply
sean Reply:
May 6th, 2010 at 12:42 am
Hey Cheryl,
Well Done! You can do it!
Have more faith in yourself on Mole Calculations.
Cheers!
Sean Chua
Reply
yap! you did it i think you can help many people in this topic
Reply
Great help.. it realy worked!
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