MadSci Network: Chemistry

Re: How many atoms of NaCl are in one grain of salt?

Date: Tue Oct 27 07:40:02 1998
Posted By: Susan Rollinson, Other (pls. specify below), organic chemistry, Alleghany Micro
Area of science: Chemistry
ID: 909364633.Ch

First letís overcome some misperceptions in the question.

NaCl is an ionic solid that strictly speaking has neither atoms nor 
molecules. It is a 3-dimensional "checkerboard" of Na+ ions and Cl- ions. 
That is, the sodium ions have one fewer electron than sodium atom have, and 
the chlorine ions have one more electron than chlorine atoms have.

So I will answer the more accurate question:
How many sodium and chlorine ions are in one grain of salt?

We need to establish how much a grain of salt weighs. For the sake of 
argument, Iíll assume that a grain of salt is 0.10 mg or 0.00010 g. (Your 
grain of salt may vary!)

We also need to look up the following information, which can be found in 
any chemistry textbook (or a general-purpose encyclopedia):
	atomic mass of sodium is 22.9898 (Iíll round to 23.0)
	atomic mass of chlorine is 35.453 (Iíll round to 35.5)
	Avogadroís number is 6.02 x 10^23

The formula mass of NaCl is 58.5 g/mol.

0.00010 g of NaCl contains 0.00010/58.5 mol =0.00000171 mol
(or 1.71 x 10^-6 mol).

Multiplying by Avogadroís number:
1.71 x 10^-6 mol * 6.02 x 10^23 formula-units/mol = 1.03 x 10^18 formula-

or, rounding to the approximate number implied in "one grain of salt":

1 x 10^18 formula-units of NaCl
1 x 10^18 Na+ ions
1 x 10^18 Cl- ions

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