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General Chemistry Principles, Patterns, and Applications, 2011

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Figure 4.7 Preparation of 250 mL of a Solution of (NH4)2Cr2O7 in Water<br />

The solute occupies space in the solution, so less than 250 mL of water are needed to make 250 mL of solution.<br />

E X A M P L E 3<br />

The solution in Figure 4.6 "Preparation of a Solution of Known Concentration Using a Solid<br />

Solute" contains 10.0 g of cobalt(II) chloride dihydrate, CoCl 2·2H 2O, in enough ethanol to make exactly 500<br />

mL of solution. What is the molar concentration of CoCl 2·2H 2O?<br />

Given: mass of solute <strong>and</strong> volume of solution<br />

Asked for: concentration (M)<br />

Strategy:<br />

To find the number of moles of CoCl 2·2H 2O, divide the mass of the compound by its molar mass. Calculate<br />

the molarity of the solution by dividing the number of moles of solute by the volume of the solution in<br />

liters.<br />

Solution:<br />

The molar mass of CoCl 2·2H 2O is 165.87 g/mol. Therefore,<br />

moles CoCl 2 •2H 2 O = ( 10 .0 g 165 .87 g /mol ) = 0 .0603 mol<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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