Question

Starting with the solid and adding water, how would you prepare \(2.00 \mathrm{~L}\) of \(0.685 \mathrm{M}\) (a) \(\mathrm{Ni}\left(\mathrm{NO}_{3}\right)_{2} ?\) (b) \(\mathrm{CuCl}_{2}\) ? (c) \(\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{6}(\) vitamin \(\mathrm{C})\) ?

Short answer

Question: What are the masses of solute needed to prepare 2.00 L of 0.685 M solution for each of the following compounds: (a) \(\mathrm{Ni}\left(\mathrm{NO}_{3}\right)_{2}\) (b) \(\mathrm{CuCl}_{2}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{6}\) (Vitamin C) Answer: The masses of solute needed for each compound are as follows: (a) 250.10 g \(\mathrm{Ni}\left(\mathrm{NO}_{3}\right)_{2}\) (b) 184.19 g \(\mathrm{CuCl}_{2}\) (c) 241.17 g \(\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{6}\) (Vitamin C)

Step-by-step solution

(Determine the moles of solute needed)

(First, we need to determine the number of moles required for the desired molarity and volume. Based on the problem, we need 2.00 L of 0.685 M solution, so the moles required can be found using the formula: moles = M * volume. Calculate the moles needed for each compound.)

(Calculate the mass of solute)

(Next, convert the number of moles of solute obtained in the previous step into grams. This can be done using the molar mass of each solute. Compute the mass for each compound.)

(Prepare the solution for each compound)

(For each compound, measure out the required mass obtained from the conversion and add it to a clean volumetric flask. Add water to the flask so that the total volume reaches 2.00 L. Mix the solution well, and the desired molarity for each compound will be achieved.) Here are the calculations for each compound: (a) For \(\mathrm{Ni}\left(\mathrm{NO}_{3}\right)_{2}\) moles = M * volume = 0.685 M * 2.00 L = 1.37 moles mass = moles * molar mass = 1.37 moles * 182.70 g/mol = 250.10 g (b) For \(\mathrm{CuCl}_{2}\) moles = M * volume = 0.685 M * 2.00 L = 1.37 moles mass = moles * molar mass = 1.37 moles * 134.45 g/mol = 184.19 g (c) For \(\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{6}\) (Vitamin C) moles = M * volume = 0.685 M * 2.00 L = 1.37 moles mass = moles * molar mass = 1.37 moles * 176.12 g/mol = 241.17 g In summary, to prepare the desired solutions, one would require 250.10 g of \(\mathrm{Ni}\left(\mathrm{NO}_{3}\right)_{2}\), 184.19 g of \(\mathrm{CuCl}_{2}\) and 241.17 g of \(\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{6}\) respectively, dissolving in water to make up a total volume of 2.00 L for each solution.

Key concepts

Molarity Calculation

Understanding molarity is the first step in preparing a solution with a specific concentration. Molarity, or molar concentration, is defined as the number of moles of a solute per liter of solution (mol/L). Calculating molarity is essential when you want to know how much substance you need to dissolve in a given volume of solvent to achieve the desired concentration.

To calculate molarity, you can use the formula:
\[ \text{Molarity (M)} = \frac{\text{moles of solute}}{\text{volume of solution in liters}} \]

In the case of the exercise, you want to prepare a 0.685 M solution. You already know the volume (2.00 L), so you can rearrange the formula to find the moles of solute needed. This forms the basis for the next steps of calculating mass and ultimately preparing the solution.

Molar Mass

In the context of preparing solutions, once you have calculated the number of moles required, you need to convert that amount into grams. To do this, you will need to know the molar mass of the substance, which is the mass in grams of one mole of the compound. It's typically found using the periodic table and adding up the atomic masses of each element in the compound.

For example, the molar mass of \(\mathrm{Ni}\left(\mathrm{NO}_{3}\right)_{2}\) is calculated by summing the atomic masses of one nickel atom, six oxygen atoms, and two nitrogen atoms. Once you have the molar mass, you translate moles into grams with the formula:
\[ \text{mass in grams} = \text{moles} \times \text{molar mass} \]

Accurate molar mass determination is crucial for precise molarity calculations, impacting the outcome of chemical reactions and studies.

Volumetric Flask Usage

A volumetric flask is a piece of laboratory glassware used for preparing precise volumes of liquid. After calculating the mass of your substance, you use a volumetric flask to ensure you add the correct amount of solvent to reach the desired volume.

In our scenario, you would add the previously weighed compound into the flask first, then add distilled water slowly. You would continue adding water until the bottom of the meniscus (the curve seen at the top of the liquid) aligns with the calibration line on the neck of the flask. It's essential to get this right, as the accuracy of the flask affects the concentration of your prepared solution.

Ensure to mix the solution thoroughly post addition of solvent, this ensures that the solute is evenly distributed throughout and contributes to the accurate preparation of a molar solution.

Stoichiometry

Stoichiometry involves the calculation of reactants and products in chemical reactions. It is a key concept in chemistry that includes the molarity calculations, molar mass evaluation, and volumetric measurements you perform while preparing solutions.

In the case of solution preparation, stoichiometry allows you to predict the outcome of a reaction by understanding the proportional relationship between reactants and products based on the balanced chemical equation. The stoichiometric coefficients in the equation indicate the molar ratios in which substances react and are formed, guiding the amount of each substance needed.

While preparing molar solutions doesn't always directly deal with reaction stoichiometry, understanding the molar relationships and conversions between moles, mass, and volume prepares students for complex applications involving reactants and products in actual chemical reactions.