The easiest way to determine the percent concentration – when you are aware of the initial mass of dry matter, subsequently diluted. Suppose initially there were 15 grams, some salt. Then it was completely dissolved in the water. You want to calculate a percent concentration.
First, weigh the container with the solution. For example, you will have 800 grams. Then pour the solution and the weight of an empty container. Suppose it weighs 550 grams. And then the problem is solved in elementary: 15/(800 – 550) = 0.06, or 6%. Here is the concentration of the solution.
A little complicate the task. Take 20 grams of salt (sodium chloride) and dissolve it in some water. Then, the overflow of the solution into the graduated cylinder and carefully add the water, bring the volume to 200 ml. What is the percentage concentration of the obtained solution?
It would seem that the task is very simple. The density of water is 1, therefore 200 ml is 200 grams, and percentage concentration is 20/200 = 0.1, or 10%. But do not rush to conclusions. Because you have not 200 milliliters of pure water, and 200 milliliters of solution, the density of which differs from unity.
So you, as in the above case, it is necessary to know the total mass of the solution M, first weigh the jar, and then – an empty vessel. Then, dividing 20 (mass of salt) M, and multiplying the result by 100%, you will receive a response.
Well, if you have some reason there is no way to weigh or measure the volume? How then to determine the percentage concentration of the solution? For example, there are a certain amount of water solution of sulfuric acid – a well-known substance. To you the task is to calculate its percentage concentration, without using any scales or measuring utensils.
At first glance, the task is very difficult. And in fact, to solve it, piece of cake. The fact that nearly any substance used in various industries, there are so-called "table of the densities of the solution". Well, sulfuric acid is such a commonly used substance that to find a table for it is not difficult. It is in most handbooks of chemistry.
First of all you need to measure the density of sulfuric acid using a device called a "densitometer" (in other words it can be called a "densitometer"). Suppose the obtained value is equal to 1.303 grams/milliliter. According to the table of densities determine: what is the solution percentage concentration corresponds to this value. This is a 40% solution of sulfuric acid. The problem is solved.