What is the molar mass of NH3? QUESTION: What is the molar mass of NH3? A. 8.00 g/mol, B. 10.00 g/mol C. 15.02 g/mol, D. 17.03 g/mol Definition: Molar mass = mass per mole Look up molar masses from periodic table. What is the molar mass of NH3? A. 8.00 g/mol, B. 10.00 g/mol C. 15.02 g/mol, D. 17.03 g/mol PAUSE CLICK The term molar mass refers to the mass of one mole of anything. Molar masses of atoms can be easily looked up from a periodic table. CLICK For example, in a typical periodic table, the average mass of nitrogen atoms in nature is given as 14.01 atomic mass units. HIGHLIGHT 14.01 A mole is defined such that the mass of one mole of nitrogen atoms is 14.01 grams. Some periodic tables may give more significant figures. For most calculations, it is usually more than sufficient to keep only four significant figures for molar masses of atoms. CLICK Similarly, the average mass of hydrogen atoms in nature is 1.008 atomic mass units. The mass in grams of one mole of hydrogen atoms is also 1.008. By now, you should know that the subscripts in the formula of a substance tell us how many moles of each element is present in the substance. In other words, we know that one mole of ammonia contains CLICK one mole of nitrogen since the subscript of nitrogen in NH3 is 1. One mole of nitrogen has a mass of 14.01 grams. Therefore, one mole of NH3 contains one mole of nitrogen, or 14.01 grams of nitrogen. CLICK Similarly, one mole of ammonia also contains 3 moles of hydrogen, since the subscript of hydrogen in NH3 is 3. HIGHLIGHT 3 in NH3 Since one mole of hydrogen has a mass of 1.008 grams, 3 moles of hydrogen will have a mass that is 3 times larger. 3 moles, times 1.008 grams per mole, gives us a total of 3.024 grams of hydrogen. In the preceding calculations, the number of significant digits in the calculated masses is 4.... the same as the number of significant digits in the molar masses that are looked up from the periodic table. The numbers 1 and 3, which are derived from the subscripts in NH3 are exact numbers. CLICK Therefore, the total mass of one mole of ammonia is 17.03 grams. We get this number by adding the mass of nitrogen and the mass of hydrogen present in one mole of ammonia. It is rounded to the nearest hundredth since the less precise term, 14.01 grams, only goes to the hundredths place. Since the total mass of one mole of ammonia is 17.03 grams, we say that its molar mass is 17.03 grams per mole. The correct answer is D. CLICK PAUSE END RECORDING One mole of NH3 contains 1 mol N: 1 mol N x 14.01 g/mol = 14.01 g N 3 mol H: 3 mol H x 1.008 g/mol = 3.024 g H Total mass of one mole of NH3 = 17.03 g

Video ID: 4-5-1 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08