Sulphuric acid , how are we prepare 2.5 normal solutions
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Answer:
The last article covered the concept of Normal solutions in the laboratory and how to calculate the equivalent mass of a compound. Then I described how to use the equivalent mass to make a solution of a predetermined Normality. However, the article did not address making Normal solutions from concentrated mineral acids like sulfuric acid, nitric acid, and hydrochloric acid. Unlike using powdered chemicals where the chemical is simply weighed out then diluted to volume, the use of liquid chemicals to make Normal solutions requires the addition of a few more calculations. This article will address these extra calculations.
First, it is important to describe a few aspects of concentrated mineral acids (as well as that of many other solutions). Most of us buy concentrated acids to use as stock solutions in the laboratory. None of these acids are one hundred percent pure. Sulfuric acid is only about 97% pure, nitric is about 69.5%, and hydrochloric acid is about 37.5% pure. Manufacturers of these acids simply cannot economically make these acids more concentrated than these respective percentages.
Another important aspect of these solutions is their specific gravities. The specific gravity of a liquid is, in most cases, synonymous with the more familiar term of density. Water has a specific gravity of 1. If the specific gravity of a liquid is greater than 1, then the liquid is heavier than water. Less than 1, and the liquid is lighter than water. The specific gravity for concentrated sulfuric acid is about 1.84, or 1.84 times heavier than an equal volume of water. The specific gravity of concentrated nitric acid is about 1.42 and that of concentrated hydrochloric acid is about 1.19.
Both the percent concentration and specific gravity values of the acid are required to determine the amount of concentrated acid needed when making a Normal solution. This information is usually printed on a label attached to the bottle of acid. Specific values vary depending on the manufacturer and lot of acid.
To make a solution of a predetermined Normality, you must first determine the equivalent mass of the chemical and then determine the grams needed of that chemical. These calculations were described in the last article, “Normality.” Then you must convert the number of grams into its volume equivalent. Once this volume is determined, it is a simple dilution after that.
Here is an example:
You want to make only 250 mL of a 1 N H2SO4 solution that will be used to adjust the pH of BOD samples prior to analysis. How many milliliters of concentrated sulfuric acid do you need to make 250 mL of a 1 N solution?
To determine how many grams of sulfuric acid you will need, you will first need to calculate the equivalent mass of H2SO4. This is the gram-formula weight divided by the number of acid hydrogens in the compound. It is 98/2 = 49.
Then you can calculate the amount of grams of H2SO4 that are needed.
The formula to calculate this is:
Grams of compound needed = (N desired)(equivalent mass)(volume in liters desired).
Substituting the above numbers into the equation, we get:
grams of compound needed = (1 N)(49)(0.250 liters) = 12.25 grams.
A 1 N solution requires 12.25 g of a pure sulfuric acid powder (if one existed) diluted to 250 mL. But the acid is a liquid and it is not one hundred percent pure active sulfuric acid. You will need to calculate what volume of the concentrated acid that contains 12.25 grams of sulfuric acid. The formula for this is:
Volume of concentrated acid needed = (grams of acid needed)/(percent concentration x specific gravity)
Continuing with the sulfuric acid example, plug into the formula the percent concentration and specific gravity from the label on the acid container. For this example, I am using those values previously mentioned in this article: volume of concentrated acid needed = (12.25 grams)/(0.97 x 1.84) = 6.9 mL
If you took 6.9 mL of concentrated sulfuric acid and diluted it to 250 mL, you would have a 1 N H2SO4 solution.
Also you can use the formula
V1S1 =V2S2 [ where V1= Primary volume, V2 = Final volume, S1 = initial specific garvity , S2 = final specific gravity]
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