Explain how you would prepare these solutions from powdered reagents and whatever glassware you needed:
a. 2.0 L of 1.5 M KOH
the answer is:
a. To prepare 2.0 L of 1.5 M KOH, measure out 168 g of KOH and place it into a 2 L volumetric flask. Add water to the mark.
BUT HOW DID THEY ARRIVE AT THIS ANSWER?
moles needed = M x L = 1.5M x 2.0L = 3.0 moles.
moles KOH = grams KOH/molar mass KOH
3.0 moles = grams/approximately 56
grams = 3.0*56 = 168 grams.
To arrive at the answer, you need to understand the concepts of molarity and volumetric flask.
1. Molarity:
Molarity (M) is a unit of concentration used to measure the amount of solute (in moles) dissolved in a given volume of solution (in liters). In this case, you are asked to prepare a 1.5 M solution of KOH.
2. Volumetric flask:
A volumetric flask is a piece of glassware used to prepare solutions with precise volumes. It has a narrow neck and a marked line that indicates a specific volume.
Now, let's break down the steps to arrive at the answer:
Step 1: Determine the molecular weight of KOH.
You need to find the molar mass of KOH, which consists of one unit of potassium (K) and one unit of hydroxide (OH). The atomic masses are K = 39.10 g/mol, O = 16.00 g/mol, and H = 1.01 g/mol. Adding these up, you get:
KOH = 39.10 + 16.00 + 1.01 = 56.11 g/mol
Step 2: Calculate the mass of KOH needed.
You are asked to prepare a 2.0 L solution with a concentration of 1.5 M.
Molarity (M) is moles of solute (KOH) per liter of solution. Therefore, you can rearrange the equation to calculate moles:
Molarity (M) = Moles (mol) / Volume (L)
Rearranging the equation gives:
Moles (mol) = Molarity (M) x Volume (L)
Plugging in the values:
Moles (mol) = 1.5 M x 2.0 L = 3.0 mol
To calculate the mass of KOH needed, you multiply the number of moles by the molar mass:
Mass (g) = Moles (mol) x Molar mass (g/mol)
Therefore:
Mass (g) = 3.0 mol x 56.11 g/mol = 168.33 g
Step 3: Prepare the solution.
Take a 2 L volumetric flask and measure out exactly 168 g of KOH. Place the KOH into the flask. Then, add water carefully while swirling the flask until the solution reaches the marked line on the neck of the flask, ensuring a total volume of 2.0 L.
By following these steps, you will prepare 2.0 L of 1.5 M KOH solution.