What is the molar mass of Ca(NO3)2?

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Presentation transcript:

What is the molar mass of Ca(NO3)2? QUESTION: What is the molar mass of Ca(NO3)2? A. 70.09 g/mol, B. 82 g/moll C. 164.10 g/mol Definition: Molar mass = mass per mole Look up molar masses from periodic table. One mole of Ca(NO3)2 contains 1 mol Ca: 1 mol Ca x 40.08 g/mol = 40.08 g 2 mol N: 2 mol N x 14.01 g/mol = 28.02 g 6 mol O: 6 mol O x 16.00 g/mol = 96.00 g Total mass of one mole of Ca(NO3)2 = 164.10 g 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 calcium atoms in nature is given as 40.08 atomic mass units. HIGHLIGHT 40.08 A mole is defined such that the mass in grams of one mole of calcium atoms is also 40.08. 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. 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 calcium nitrate contains CLICK one mole of calcium since the subscript of calcium in the formula is implied to be 1. One mole of calcium has a mass of 40.08 grams. Therefore, one mole of calcium nitrate contains one mole of calcium, or 40.08 grams CLICK Similarly, one mole of calcium nitrate also contains 2 moles of nitrogen The subscript of nitrogen in nitrate is implied to be 1. But there are two nitrates in the formula HIGHLIGHT 2 in Ca(NO3)2 Therefore, one mole of this compound also contains 2 moles of nitrogen. If we look up the molar mass of nitrogen, we find that it is 14.01 grams per pmole. ... 2 moles of nitrogen times 14.01 grams per mole... is 28.02 grams. One mole of this compound contains 28.02 grams of nitrogen. Finally, one mole of this compound also contains 6 moles of oxygen. The subscript of oxygen in nitrate is 3. HIGHLIGHT 3 and there are two nitrates in the formula. HIGHLIGHT 2 Therefore, one mole of this compound also contains 6 moles of oxygen. If we look up the molar mass of oxygen, we find that it is 16.00 grams per mole 6 moles times 16.00 grams per mole equals 96.00 grams Note that in the preceding calculations, the number of moles derived from the subscripts are exact numbers. HIGHLIGHT 1, 2 and 6. So the calculated masses retain the same number of significant figures as the molar masses that we looked up from the periodic table. They are all expressed to four significant figures. CLICK Adding all of these masses, we get a total mass of 164.10 grams for one mole of calcium nitrate. Therefore, the molar mass of calcium nitrate is 164.10 grams per mole. The correct answer is C. CLICK PAUSE END RECORDING

Video ID: 4-5-2 © 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