A solution of ethanol (C2H5OH) in water is prepared by dissolving 57.0 mL of ethanol (density = 0.79 g/cm3) in enough water to make 240.0 mL of solution. What is the molarity of the ethanol in this solution?
mass = volume x density. Use this to calculate the mass of ethanol.
mols ethanol = grams/molar mass.
M = mols/L = mols ethanol/0.240 L = ??
To find the molarity of the ethanol in the solution, we need to know the moles of ethanol present and the volume of the solution.
First, let's calculate the mass of the ethanol used. To do this, we multiply the volume of ethanol (57.0 mL) by its density (0.79 g/cm³):
Mass of ethanol = Volume of ethanol × Density of ethanol
= 57.0 mL × 0.79 g/cm³
Note: We need to convert the units of mL to cm³ because the density is given in g/cm³.
But, 1 mL = 1 cm³, so the mass of ethanol is also equal to:
Mass of ethanol = 57.0 g
Next, let's calculate the number of moles of ethanol using its molar mass. The molar mass of ethanol (C₂H₅OH) is the sum of the atomic masses of its constituent elements:
C = 12.01 g/mol
H = 1.01 g/mol (there are 6 hydrogen atoms in ethanol)
O = 16.00 g/mol
Molar mass of ethanol = (2 × C) + (6 × H) + O
= (2 × 12.01) + (6 × 1.01) + 16.00 g/mol
Now, calculate the moles of ethanol:
Moles of ethanol = Mass of ethanol / Molar mass of ethanol
= 57.0 g / [(2 × 12.01) + (6 × 1.01) + 16.00] g/mol
Once we know the moles of ethanol, we can find the molarity using the formula:
Molarity (M) = Moles of solute / Volume of solution in liters
In this case, the volume of the solution is given in milliliters (mL), so we need to convert it to liters:
Volume of solution = 240.0 mL = 240.0/1000 L
Finally, we can calculate the molarity:
Molarity of ethanol = Moles of ethanol / Volume of solution
= Moles of ethanol / (240.0/1000) L
By following these steps and plugging in the appropriate values, you can calculate the molarity of the ethanol in the solution.