how many grams of potassium dichromate are needed to make 250 mL of a 0.75 molar solution?

How many moles do you need? M x L = mols.

moles dichromate = grams dichromate/molar mass dichromate. Solve for grams.

To determine the number of grams of potassium dichromate needed to make a 0.75 molar solution, you'll need to follow a few steps.

Step 1: Understand the Problem
A molar solution means that there is 1 mole of the solute (in this case, potassium dichromate) dissolved in 1 liter (1000 mL) of the solvent (in this case, water). We want to prepare a 0.75 molar solution using 250 mL of water.

Step 2: Convert mL to Liters
Since the molar concentration is given in liters, we need to convert 250 mL to liters by dividing 250 by 1000:
250 mL ÷ 1000 = 0.25 L

Step 3: Calculate the Number of Moles
To find the number of moles (n) of potassium dichromate needed, we can use the formula:
n = molarity × volume in liters

Substituting the values given:
n = 0.75 mol/L × 0.25 L
n = 0.1875 moles of potassium dichromate

Step 4: Convert Moles to Grams
To convert from moles to grams, we need to know the molar mass of potassium dichromate. The molar mass is obtained from the atomic masses of its constituent elements, which can be found on the periodic table.

Potassium (K) atomic mass = 39.10 g/mol
Dichromate (Cr2O7) atomic mass = 52.0 g/mol

Now, calculate the molar mass of potassium dichromate:
Molar mass of K2Cr2O7 = 2 × (potassium atomic mass) + (chromium atomic mass) + 7 × (oxygen atomic mass)
Molar mass of K2Cr2O7 = 2 × 39.10 g/mol + 52.0 g/mol + 7 × 16.00 g/mol = 294.18 g/mol

Finally, calculate the mass (m) of potassium dichromate needed by using the formula:
m = n × molar mass

Substituting the values obtained:
m = 0.1875 moles × 294.18 g/mol
m ≈ 55.17 grams (rounded to two decimal places)

Therefore, approximately 55.17 grams of potassium dichromate are needed to make 250 mL of a 0.75 molar solution.