Moles, volume and
concentrations of chemical solutions.
1 dm3 = 1000 cm3
250 cm3 = 0.250 dm3
10.0 cm3 = 0.0100 dm3
So how do we get the number of moles
when the chemical is dissolved in water?
Concentration is expressed as
moles per cubic decimetre.
moldm-3.
TO LOWER CONCENTRATION
RINSINGS: To rinse the empty beaker, glass rod and funnel to ensure that all of the solute in transferred to the volumetric flask.
See related image here.
Preparing a standard solution.
Defined as a solution of known concentration.
How would we prepare 250.0 cm3 of copper(II) sulfate solution with a concentration of 0.80 moldm-3 ?
Hydrate copper(II) sulfate [CuSO4.5H2O] and distilled water are available.
- Use the formule n = c.v to calculate the number of moles required.
250 cm3 is 0.250 dm3. n = 0.80 x 0.250 = 0.200 mol CuSO4.5H2O. - Calculate the mass of solid that contains this number of moles.
The rfm is 249.6 and mass = rfm x mol = 249.6 x 0.200 = 49.92 g - Weigh this mass into a beaker, using a top pan balance.
- Dissolve the solid completely in deionsed water.
- Transfer the solution to a 250 cm3 volumetric flask (including rinsings).
- Make the volume up to just below the mark using distilled water.
- Use a dropper with distilled water to bring the meniscus to the mark.
- Stopper, invert the flask several times and label.
Question 2.
Calculate the mass of ammonium chromate(VI) present in 25.0 cm3 of a solution of concentration 2.00 moldm-3. The rfm is 152.0.
Did you get 7.60 g ? The solution is here.
Question 4.
Determine the rfm of a solute given that 30.0 cm3 of a solution of concentration 0.0500 moldm-3 contains 0.44 g.
Did you get 293? The solution is here.
Question 5.
You are provide with a stock solution of barium phosphate of concentration 1.00 moldm-3. How would you use this solution to prepare 25.0 cm3 of a solution of concentration 0.200 moldm-3.
(rfm = 152)
Did you say to take 5.00 cm3 of the stock solution and make it up to
25.0 cm3 with distilled water ? The solution is here.
Question 7.
25.0 cm3 of a copper(II) sulfate solution of concentration 2.00 moldm-3 was boiled until saturated. It was then left for the solute to completely crystallise as hydrated copper(II) sulfate.
Calculate the maximum mass of crystals that could form.
Did you get 12.5 g ? The solution is here.
Question 8.
Limewater is a saturated solution of calcium hydroxide. However, it is only slightly soluble in water. At 20 oC 0.173 g dissolves in 100 g of water.
Determine the concentration of limewater at 20 oC. (rfm = 74.1)
Did you get 0.0231 moldm-3 ? The solution is here.
Sometimes the concentration is expressed in gdm3.
- convert moles to mass by multiplying by the r.f.m.
EXAMPLE.
The rfm of strontium chloride (SrCl2) is 158.6.
If a solution of this salt has a concentration of 0.250 moldm-3,
calculate its concentration in gdm-3.
= 0.250 x 128.6 = 32.15 gdm-3.
Question 9.
The rfm of manganese(II) sulfate is 151.0.
The solubility of the salt at 20oC is 70.0 g per 100 g of water. Calculate the concentration of a saturated solution of manganese sulfate at 20 oC, in gdm-3.
Did you get 700 gdm-3 ? The solution is here.
