Question

Which of the following do you need to know to calculate the molarity of a salt solution? (There may be more than one answer.) a. the mass of salt added b. the molar mass of the salt c. the volume of water added d. the total volume of the solution Explain your answer.

Short answer

To calculate the molarity of a salt solution, you need to know the following: a. the mass of salt added - necessary to calculate the number of moles of solute (salt). b. the molar mass of the salt - also necessary to calculate the number of moles of solute. d. the total volume of the solution - this is crucial because molarity is defined as the number of moles of solute per liter of solution. The volume of water added (c) is not necessary for the calculation as molarity is dependent on the total volume of the solution, not the volume of solvent (water, in this case).

Step-by-step solution

Understanding Molarity

Molarity is defined as the number of moles of solute (e.g., salt) per liter of solution. The molarity can be represented as \(M\) and the formula used is: \[M = \frac{\text{moles of solute}}{\text{volume of solution (in liters)}}\]

Moles of Solute

To determine the number of moles of solute, we need to know the mass of salt added (a) and the molar mass of the salt (b). The number of moles can be calculated using this formula: \[\text{moles of solute} = \frac{\text{mass of salt added}}{\text{molar mass of the salt}}\]

Determining Volume of Solution

The volume of the solution is the measure of how much space the solution occupies. To find this, we need to know the total volume of the solution (d), and it must be in liters for the molarity calculation.

Answer and Explanation

Based on the analysis above, we can conclude that to calculate the molarity of a salt solution, we need to know: a. the mass of salt added b. the molar mass of the salt d. the total volume of the solution The volume of water added (c) is not required to calculate molarity, as molarity is based on the total volume of the solution, not just the volume of the solvent.

Key concepts

Molarity Definition

Molarity is a core concept in chemistry that reflects the concentration of a solution. Simply put, it measures how much of a substance is dissolved in a given volume of solvent. Defined in more formal terms, molarity is the number of moles of solute per liter of solution, which can symbolically be represented as 'M'.

When you come across a problem that asks you to calculate molarity, you're working out the strength of the solution. This is expressed mathematically by the formula: \[M = \frac{\text{moles of solute}}{\text{volume of solution (in liters)}}\] Understanding this formula is vital for students tackling problems involving solution concentration, especially when studying reactions that take place in an aqueous medium.

For example, if you dissolve sugar in water to make a sugary drink, the molarity tells you how sugary the drink is. In a laboratory setting, scientists and students alike use molarity for making precise solutions for experiments.

Moles of Solute

The mole is a fundamental unit in chemistry, serving as a bridge between the microscopic world of atoms and molecules and the macroscopic world we observe. The moles of a solute in a solution represent how many units of that substance are present. To determine this, two pieces of information are required: the mass of the solute that has been added to the solution, and its molar mass, which is its weight per mole typically expressed in grams per mole (g/mol).

You can calculate the number of moles of solute using this formula: \[\text{moles of solute} = \frac{\text{mass of salt added}}{\text{molar mass of the salt}}\] For instance, if you have 58.44 grams of table salt (sodium chloride, NaCl) and you know NaCl has a molar mass of 58.44 g/mol, you'll find that you added exactly 1 mole of salt to your solution. A clear understanding of moles allows students to precisely calculate how much substance to use for a specific molar concentration.

Total Volume of Solution

In the context of molarity, the 'total volume of solution' refers to the entire amount of space that the solution—not just the solvent—occupies. It's crucial to comprehend that this includes both the solute and the solvent combined. The total volume is always measured in liters when calculating molarity, as the standard equation necessitates.

It is not the volume of water or solvent added that we need to know, but the final volume of the solution as a whole, which is determined after the solute has been mixed with the solvent. This volume is what we plug into the molarity formula: \[M = \frac{\text{moles of solute}}{\text{volume of solution (in liters)}}\] For practical calculations, if you dissolve the solute into a solvent, you would typically measure the total volume of the resulting solution to determine its concentration, not just the amount of solvent you started with. This concept is integral to preparing solutions with precise molar concentrations for various chemical applications or experiments.