Question

You have two solutions of chemical A. To determine which has the highest concentration of A (molarity), which of the following must you know (there may be more than one answer)? a. the mass in grams of $\mathrm{A}$ in each solution b. the molar mass of $\mathrm{A}$ c. the volume of water added to each solution d. the total volume of the solution Explain.

Short answer

To determine which solution has the highest concentration of chemical A (molarity), you must know the mass in grams of chemical A in each solution (option a), the molar mass of chemical A (option b), and the total volume of the solution (option d). Knowing the volume of water added to each solution (option c) is not sufficient on its own to determine the concentration.

Step-by-step solution

Option A: Mass in grams of chemical A

To know the number of moles of chemical A in each solution, it is necessary to know the mass in grams of chemical A present. This information on its own cannot give us the concentration, but it is crucial for calculating the number of moles. So, this is a piece of required information.

Option B: Molar mass of chemical A

The molar mass of chemical A is the mass of one mole of the substance. To calculate the number of moles of chemical A in each solution, we need to divide the mass of chemical A by its molar mass. Therefore, knowing the molar mass of chemical A is essential to determine the concentration.

Option C: Volume of water added to each solution

Knowing the volume of water added to each solution is not enough, as it does not give us the total volume of the solution. The concentration of solute depends upon the total volume of the solution, including both solute and solvent. Therefore, this information is not enough to determine the concentration of chemical A.

Option D: Total volume of the solution

Knowing the total volume of the solution is necessary because molarity is defined as the number of moles of solute per liter of solution. To determine the concentration of chemical A in each solution, we need to know the total volume of the solution. So, this information is required. Based on the analysis, options a, b, and d are needed to determine which solution has the highest concentration of chemical A (molarity). Option c, the volume of water added to each solution, is not sufficient on its own to determine the concentration.

Key concepts

Molar Mass

Molar mass is an essential concept in chemistry that defines the mass of one mole of a given substance, typically expressed in grams per mole (g/mol). It plays a crucial role when you need to determine the amount of a substance present in a chemical reaction or solution.
To find the number of moles in a given amount of a substance, you'll divide the mass of the substance by its molar mass. This relationship is fundamental for calculating concentrations and quantities in chemical equations.
- **Example**: If the substance is sodium chloride (NaCl), and its molar mass is approximately 58.44 g/mol, - To find how many moles are in a 100 g sample, divide 100 g by 58.44 g/mol. This gives you about 1.71 moles.
Understanding molar mass allows you to convert between the mass of a substance and the amount in moles, which is invaluable for preparing solutions with precise concentrations.

Concentration

Concentration is a measure of how much solute is dissolved in a given volume of solution. A commonly used unit for concentration is molarity, represented as moles per liter (M). Understanding concentration is crucial when preparing solutions or comparing the strength of different solutions.
When you want to determine the molarity of a solution, you must know:- The number of moles of solute present. - The total volume of the solution in liters.
The formula to calculate molarity ($M$) is: $$M=\frac{\text{moles of solute}}{\text{volume of solution in liters}}$$
Higher molarity indicates a higher concentration of solute in the solution. This basic concept of molarity helps one understand not just the solution's strength but also its potential reactivity and behavior in different conditions.

Moles of Solute

Moles represent a standard unit of measurement in chemistry that describes an amount of a chemical substance. One mole equates to Avogadro's number, approximately $6.022\times {10}^{23}$ entities, typically atoms or molecules.
To find the number of moles in a given sample, you need the mass of your solute and its molar mass. Using the relationship:
$$\text{moles of solute}=\frac{\text{mass of solute in grams}}{\text{molar mass in g/mol}}$$
Calculating moles allows you to determine how many particles are involved in a reaction or how they are distributed within a solution. This fundamental understanding is not only essential for calculating concentration but also for stoichiometry and chemical reaction predictions.

Solution Volume

The volume of a solution is vital when determining the concentration of a solute within that solution. Solution volume typically refers to the total volume, including both the solute and solvent.
It's measured in liters or milliliters, depending on the concentration needed.
Total solution volume is necessary for calculating molarity because it provides the context for how dispersed the solute particles are. The relationship between moles and volume forms the basis of the concentration formula:
$$M=\frac{\text{moles of solute}}{\text{solution volume in liters}}$$
Knowing the exact volume ensures accurate molarity calculations, which is crucial for experiments and practical applications such as pharmaceuticals and chemical manufacturing.