Understanding Molarity: How to Calculate Molarity Given Volume

Molarity is one of the most important concepts in chemistry, particularly when dealing with solutions. It is defined as the number of moles of solute per liter of solution. However, a common question arises: if you are given only the volume of a solute, such as 10 cm3 of sulfuric acid, can you determine its molarity? The answer is no, because molarity requires information on both the volume and the concentration of the solution.

Introduction to Molarity

Molarity is a measure of the concentration of a solution and is given by the formula:

Molarity (M) Moles of Solute / Liters of Solution

To use this formula, you need to know the number of moles of solute and the volume of the solution in liters. Without either piece of information, it is impossible to calculate molarity.

The Pointless Nature of the Given Question

For example, if you ask, 'What is the molarity of 10 cm3 of sulfuric acid?' this question lacks the necessary information to provide an answer. You might be considering a concentration of 18 M, 1 M, or 0.05 M, but without knowing the specific concentration, it is impossible to determine the molarity accurately.

Examples and Calculations

A more appropriate question might be: 'If I add 10 ml of 95% H2SO4 (sulfuric acid) and dilute it to 100 ml, what is the molarity of the resulting solution?' Here’s how you can approach this problem:

Convert the volume of sulfuric acid from ml to liters: 10 ml 0.01 L Convert the percentage of sulfuric acid to molarity: The molarity of 95% sulfuric acid is approximately 17.5 M (this can be found on data sheets or through calculations). Calculate the moles of sulfuric acid: Moles of H2SO4 Molarity × Volume 17.5 M × 0.01 L 0.175 moles Calculate the final molarity after dilution: The total volume of the solution after dilution is 100 ml or 0.1 L. Therefore, the molarity of the diluted solution is: M Moles of Solute / Volume of Solution 0.175 moles / 0.1 L 1.75 M

Another Example for Clarification

Consider another scenario: 'If I need 250 ml of a 2 M H2SO4 solution, how much volume of a 18 M H2SO4 bottle do I need?' Here’s the step-by-step calculation:

Determine the moles of sulfuric acid required for the 2 M solution: Moles of H2SO4 Molarity × Volume 2 M × 0.250 L 0.5 moles Calculate the volume of 18 M sulfuric acid needed: Volume of H2SO4 Moles of Solute / Molarity 0.5 moles / 18 M 0.0278 L or 27.8 ml

Conclusion

Given the above examples, it is clear that to determine the molarity of a solution, you must know both the volume of the solution and its concentration. Without either of these pieces of information, it is impossible to calculate the molarity accurately. Understanding these principles is crucial when working with chemical solutions in the laboratory and beyond.