Determining the Amount of Reagent Needed for Solution Preparation

Before you prepare a solution you must calculate the correct amount of reagent that is to be diluted to make the solution. Several examples of how to make such calculations are given below:

Approximate Dilution of a Concentrated Reagent

What volume concentrated nitric acid is needed for the preparation of 100 mL of 6।0 M HNO3? The calculation is as follows:



Use a graduated cylinder to measure 38 mL of concentrated nitric acid and add this amount to approximately 60 mL of distilled water in a plastic bottle. Mix thoroughly, let the solution cool, then cap and label the container.

Finding the molarity

If the molarity of the reagent is not known, you must find it from the given value of specific gravity and percent composition। For all practical purposes, the specific gravity of a solution is numerically equal to its density। For example, what is the molarity of concentrated nitric acid given that it is 70% HNO3 by mass and has a specific gravity of 1.42?

First, calculate the mass of one liter of solution:


The solution is 70% by mass, so 100 g of solution contains 70 g of pure HNO3. The mass of HNO3 in one liter of concentrated solution is:


To find the number of moles in one liter of solution (the molarity):

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Quantitative Dilution of a Standard Reagent

A common calculation involved in solution preparation is the one used to make the correct dilution of a more concentrated solution to prepare a new. For example, what volume of 2.00 M HCl solution is needed to prepare a 100-mL solution that is 0.100 M?

The solution is then prepared by pipetting 5.00 mL of 2.00 M HCl solution and diluting it in a 100-mL volumetric flask as described previously.

Preparation of a Solution from a Solid Suppose a you wish to prepare one liter of a 20.0% (w/v) sodium chloride solution. Find the amount of NaCl needed.

"The percentage of mistake in quick decisions is no greater than in long-drawn-out vacillations, and the effect of decisiveness itself makes things go and creates confidence." --Anne O'Hare McCormick