Question

How many moles of solute particles are present in \(1 \mathrm{~L}\) of each of the following aqueous solutions? (a) \(0.3 M \mathrm{KBr}\) (b) \(0.065 \mathrm{M} \mathrm{HNO}_{3}\) (c) \(10^{-4} M \mathrm{KHSO}_{4}\) (d) \(0.06 M\) ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\)

Short answer

(a) 0.6 moles (b) 0.13 moles (c) 2 x 10⁻⁴ moles (d) 0.06 moles

Step-by-step solution

Understanding the concept

To find the number of moles of solute particles in a solution, we need to consider the dissociation in water if applicable. The molarity (M) of a solution indicates the number of moles of solute per liter of solution.

Calculation for (a) \(0.3 M \mathrm{KBr}\)

Potassium bromide (KBr) dissociates completely in water into K⁺ and Br⁻ ions.Total moles of ions produced = \(1 \mathrm{K}^{+} + 1 \mathrm{Br}^{-} = 2 \text{ions}\). Thus, \(0.3 M \mathrm{KBr}\) solution has \(0.3 \times 2 = 0.6 \text{moles of ions}\).

Calculation for (b) \(0.065 \mathrm{M} \mathrm{HNO}_{3}\)

Nitric acid (HNO₃) dissociates completely in water into H⁺ and \mathrm{NO}_{3}^{-}\ ions.Total moles of ions produced = \(1 \mathrm{H}^{+} + 1 \mathrm{NO}_{3}^{-} = 2 \text{ions}\). Thus, \(0.065 M \mathrm{HNO}_{3}\) solution has \(0.065 \times 2 = 0.13 \text{moles of ions}\).

Calculation for (c) \(10^{-4} \mathrm{M} \mathrm{KHSO}_{4}\)

Potassium bisulfate (KHSO₄) dissociates in water into \mathrm{K}^{+}\ and \mathrm{HSO}_{4}^{-}\ ions.Total moles of ions produced = \(1 \mathrm{K}^{+} + 1 \mathrm{HSO}_{4}^{-} = 2 \text{ions}\). Thus, \(10^{-4} M \mathrm{KHSO}_{4}\) solution has \(10^{-4} \times 2 = 2 \times 10^{-4}\ \text{moles of ions}\).

Calculation for (d) \(0.06 M \text{ethanol} \left(\text{C}_{2} \text{H}_{5} \text{OH}\right)\)

Ethanol (C₂H₅OH) does not dissociate in water. Therefore, the number of moles of solute particles remains the same as its molarity.Thus, \(0.06 M \text{ethanol}\) solution has \(0.06 \text{moles of solute particles}\).

Key concepts

Molarity

Molarity is a key concept in chemistry. It measures the concentration of a solute in a solution. The unit for molarity is moles per liter (M), which tells us how many moles of a solute are in one liter of solution. For example, a 0.3 M solution of KBr means there are 0.3 moles of KBr in every liter of the solution.
Calculating molarity helps determine how concentrated or diluted a solution is. Knowing this is essential for various applications in chemistry, such as preparing solutions, conducting reactions, and even understanding biological processes.
In simple terms, think of molarity as a way to express how 'packed' a solution is with its solute.

Dissociation in Water

When certain substances dissolve in water, they can split into ions. This process is called dissociation. For instance, KBr (potassium bromide) dissociates into K⁺ (potassium ions) and Br⁻ (bromide ions) when it dissolves in water.
Each molecule of KBr forms two ions. So, if you have a solution with 0.3 M KBr, it correlates to 0.6 moles of ions (0.3 moles of K⁺ and 0.3 moles of Br⁻). This dissociation is crucial in calculating the total number of solute particles in a solution.
However, not all substances dissociate. Ethanol (C₂H₅OH) is an example where no dissociation occurs. For ethanol in a 0.06 M solution, it remains 0.06 moles of solute particles since it doesn't split into ions.

Solute Particles

Understanding solute particles is essential for solving chemistry problems involving solutions. Solute particles refer to the individual molecules or ions in a solution. The number of these particles can change depending on whether the solute dissociates in water.
In a 0.065 M HNO₃ (nitric acid) solution, HNO₃ dissociates into H⁺ and NO₃⁻ ions. Each HNO₃ molecule becomes two ions, making the total concentration of particles 0.13 moles.

• For substances that dissociate completely, you must consider the number of particles formed.
• For substances that do not dissociate, the number of solute particles equals the molarity of the solution.

This understanding helps you determine the particle count in solutions, which is vital for predicting how reactions will proceed and for performing precise measurements in lab settings.