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When you calculate the mass of borax needed to make up a standard solution, do you include $\ce{\cdot 10 H2O}$ in the formula for borax? Why or why not?

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  • $\begingroup$ In general, you include water when it is there, and you don't when it is not, much like you would do with anything else in a chemical formula. That dot ($\cdot$) in the formula does not (I repeat, does not) make everything after it "insignificant" or "kinda not real". $\endgroup$ Commented Apr 22, 2016 at 21:00

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You are talking about the waters of hydration in crystal structure $\ce{Na2B4O7 \cdot 10H2O}$, and the answer is invariably—yes.

These water molecules account for 47.24% of the crystals weight! If you wanted to make $1\: \mathrm{L}$ of $1\: \mathrm{M}$ of $\ce{Na2B4O7}$, and you did not account for these waters of hydration, you would end up making a $0.4818\: \mathrm{M}$ solution. Less than half the desired concentration!

This is the result of a double effect. Not only are you not adding as much salt as you thought you were, but because the salt contains water it is diluting the solution even further. In the case above $95.05\: \mathrm{mL}$ of water are actually added to the solution just in the salt!

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