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A chemist prepares a solution by adding 424 mg of CoCl2 (MW = 129.84 g/mol ) to a volumetric flask, and then adding water until the total volume of the contents of the flask reaches the calibration line that indicates 500 mL . Determine the molarity of the prepared solution.

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Answer:

  • 0.00654 M

Explanation:

1) Data:

a) mass = 422 mg

b) MW = 129.84 g/mol

c) V = 500 ml

d) M = ?

2) Chemical principles and formulae:

a) Molarity a the unit of concentration that measures the number of moles of solute per liter of solution:

  • M = number of moles of solute / V in liter

b) The number of moles may be calculated by this formula:

  • number of moles = mass in grams / molar mass

3) Calculations:

a) Unit conversions

  • mass in grams = 424 mg × 1 g / 1,000 mg = 0.424 g

  • Volume in liters = 500 ml × 1 liter / 1,000 ml = 0.500 ml

b) Number of moles of solute, n:

  • n = mass in grams / molar mass = 0.424 g / 129.84 g/ mol = 0.00327 mol

c) Molarity, M:

  • M = n / V in liters = 0.00327 mol / 0.500 liter = 0.00654 M ← answer

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