What is the molarity of a solution composed of 8.210 g of potassium chromate, K2CrO4 (molar mass 194.20 g/mol), dissolved in enough water to make 0.500 L of solution?
The correct answer and explanation is:
Correct Answer:
0.0845 M
Explanation:
To find the molarity of a solution, begin by recalling that molarity (M) is defined as the number of moles of solute per liter of solution. The solute in this case is potassium chromate, K₂CrO₄. The first step is to calculate how many moles of K₂CrO₄ are present in the given mass.
Start by using the molar mass. Potassium chromate has a molar mass of 194.20 g/mol. With a given mass of 8.210 grams, the number of moles is found by dividing the mass by the molar mass:
Moles of K₂CrO₄ = 8.210 g ÷ 194.20 g/mol
Moles of K₂CrO₄ = 0.0423 mol
Next, use the definition of molarity. The solution’s volume is given as 0.500 liters. Molarity is calculated as:
Molarity (M) = Moles of solute ÷ Liters of solution
Molarity (M) = 0.0423 mol ÷ 0.500 L
Molarity (M) = 0.0845 M
This means that each liter of this potassium chromate solution would contain 0.0845 moles of K₂CrO₄.
Understanding this calculation helps develop a strong grasp of solution concentration concepts. Molarity is important in stoichiometry because it connects the mass of a substance to the volume of solution used in a reaction. When preparing a solution, precise measurement of solute mass and solvent volume is essential. If either is inaccurate, the calculated molarity will not reflect the true concentration, potentially leading to incorrect results in any reactions where the solution is used.
In practical laboratory settings, chemists often use molarity to determine how much reactant is required or how much product might form. For example, if a reaction calls for a specific number of moles of K₂CrO₄, knowing the molarity allows the chemist to measure the exact volume of solution needed. This avoids waste and ensures accurate chemical reactions.