38. a sample of calcium chloride, cacl2, has a mass of 27.8 g (ref: p.330-340)
mokes of compound
a) how many moles of calcium chloride are in this mass? show all steps and work leading to your
final answer.
b) how many chloride ions are present? show all steps and work leading to your final answer.

Explanation:

Given parameters:

Mass of CaCl₂ = 27.8g

Unknown:

Number of CaCl₂ in the compound = ?

Number of Cl⁻ ions in the compound = ?

Solution:

Problem 1:

The mole is the quantitative unit of measuring particles. The mole is defined as the amount of substance. It is usually expressed as:

         Number of moles =     (1)

To find the number of moles of CaCl₂, we have been given the mass of CaCl₂  but we don't know the molar mass of the compound yet.

  Molar mass of CaCl₂ = 40 + 2(35.5) = 111g/mol

Since mass of CaCl₂  is 27.8g, we then substitute in the equation (1):

    Number of moles =   = 0.2505mole

Problem 2:

     To find the number of chloride ions present in the compound, we must express the compound in its ionic form:

                            CaCl₂ ⇄ Ca²⁺ + 2Cl⁻  

This is the ionic form of the compound when in aqueous solutions.

In this kind of problem, we solve from the specie with the known parameters. Here, we know the number of moles of CaCl₂. We can use it determine the number of Chloride ions.

From the ionic form, we see that:

      1 mole of the known CaCl₂ produced 2 moles of the unknown Cl⁻

            0.2505 mole of CaCl₂ will produce (2 x 0.2505)moles of Cl⁻

This gives, 0.5009 mole of Cl⁻ ions in the CaCl₂ compound.

Note:   1 mole of a substance gives 6.02 x 10²³particles

             0.5009mole of Cl⁻ will produce 0.5009 x 6.02 x 10²³;

                                    =  3.02 x 10²³ chloride ions are present

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