- - graduated measuring cup;
- - laboratory scales;
- - glass pipette;
- - litmus;
- - alkaline solution.
Suppose you have a container with a label H2SO4.That is, once it becomes clear: it contains sulfuric acid.But there is no more information.How do we determine its concentration ?You can do this with a spreadsheet solution density.There is a plurality of guides, where the values of the density of the solution of chemical substances, depending on their concentration.
Take a graduated measuring cup, weigh it on a laboratory scale.Designate a lot of empty cups as m1.Using a glass pipette tide in a certain volume V of sulfuric acid .Again weigh cup, indicate its mass as m
2.Density acid is given by: (m2 - m1) / V.
In the table of densities set concentration of the solution.Suppose, in the described experience you calculate the density of sulfuric acid : 1,303 grams / milliliter.It corresponds to 40% of th concentration.
How else determined by the concentration of acid ?There is a very sensitive and accurate method called direct titration.It is based on the neutralization reaction acid alkali solution whose concentration is known.For example, in the case of sulfuric acid: 2NaOH + H2SO4 = Na2SO4 + 2H2O.
reaction formula shows that the neutralization of one mole of acid need two moles of sodium hydroxide.Based on this, knowing the volume of the sample solution acid , the amount of alkali that went to neutralize it, and concentration of alkali, and can calculate the concentration of acid .
But how to determine the exact amount of alkali required to neutralize the acid ?With the help of the indicator to change color.For example, litmus.The experiment was carried so.Above the vessel with a known volume of acid (which also add a few drops of the indicator) secure a graduated burette with alkali solution.
Record the top-level alkali, followed by gently unscrewing the valve burettes, drop by drop begin to surge its acid.Your task - to turn off the tap at a time when the red color of the indicator disappears.Record the lower level of alkali and calculate what its scope left for neutralizing acid .
And then, knowing the value of the volume and exact concentration of alkali, can easily calculate how many moles of alkali react.Accordingly, the number of moles of acid was 2 times less.Knowing the initial volume acid , you will find it molar concentration .