How To Solve Mole Problems

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Understanding the relationship between the masses of atoms and the chemical formulas of compounds allows us to quantitatively describe the composition of substances.

In an earlier chapter, we described the development of the atomic mass unit, the concept of average atomic masses, and the use of chemical formulas to represent the elemental makeup of substances.

Sodium chloride is an ionic compound composed of sodium cations, Na Note that the average masses of neutral sodium and chlorine atoms were used in this computation, rather than the masses for sodium cations and chlorine anions.

This approach is perfectly acceptable when computing the formula mass of an ionic compound.

For example, if you have mixed 25g of table salt, or Na Cl, into 2 liters of water, you first need to determine how many moles are in 25g of Na Cl.

A published author and professional speaker, David Weedmark was formerly a computer science instructor at Algonquin College., is a covalent compound and the active ingredient in several popular nonprescription pain medications, such as Advil and Motrin.What is the molecular mass (amu) for this compound? 1 mole of a pure substance is defined as having a mass in grams equal to its relative molecular mass.So, mass of 1 mole of a pure substance = relative molecular mass in grams And, mass of 1 mole of a pure substance = molar mass of the pure substance (g mol From the table we see that 1 mole of water has a mass of 18.016 grams, which isn't very much (about the mass of water in a couple of small ice-cubes you'd make in your family freezer). If 1 mole of water has a mass of 18.016 g, then ½ mole of water must have ½ the mass: mass of ½ mole of water = ½ × mass of 1 mole of water mass of ½ mole of water = ½ × 18.016 = 9.008 g In both of the examples above, we can calculate the mass of water in grams by multiplying the moles of water by the mass of 1 mole of water in grams: mass water = moles of water × mass of 1 mole water because the mass of 1 mole of water in grams is known as its molar mass, we can write: mass water = moles of water × molar mass of water The table below compares the mass of different amounts of water in moles and the data is graphed on the right: From the data in the table we can generalise and say that for any pure substance the mass of substance in grams is equal to the moles of substance multiplied by the mass of 1 mole of the substance: mass = moles × mass of 1 mole and since mass of 1 mole of a substance (in grams) = molar mass (in grams per mole) mass (g) = moles × molar mass (g mol What if you knew the amount of a pure substance in moles and its mass? Recall that mass = moles × molar mass or m = n × M (a) We could use some algebra: divide both sides of the equation by the moles: molar mass = mass ÷ moles M = m ÷ n (b) We could use some logic: By inspection of units we see that dividing the mass in grams by the amount in moles we arrive at a quantity with the units grams per mole (g mol, is an important industrial chemical.For further information, including about cookie settings, please read our Cookie Policy .By continuing to use this site, you consent to the use of cookies.Solution Molecules of this compound are comprised of 13 carbon atoms, 18 hydrogen atoms, and 2 oxygen atoms.Following the approach described above, the average molecular mass for this compound is therefore: Check Your Learning Acetaminophen, C Ionic compounds are composed of discrete cations and anions combined in ratios to yield electrically neutral bulk matter.Consider chloroform (CHCl), a covalent compound once used as a surgical anesthetic and now primarily used in the production of the “anti-stick” polymer, Teflon.The molecular formula of chloroform indicates that a single molecule contains one carbon atom, one hydrogen atom, and three chlorine atoms.


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