What is the mass of 2.00 mol NH3? A g, B g, C g

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Presentation transcript:

What is the mass of 2.00 mol NH3? A. 20.0 g, B. 34.1 g, C. 30.0 g QUESTION: What is the mass of 2.00 mol NH3? A. 20.0 g, B. 34.1 g, C. 30.0 g Molar mass of NH3: 17.03 g/mol mass NH3 17.03 g NH3 = moles NH3 1 mol NH3 What is the mass of 2.00 mol NH3? A. 20.0 g, B. 34.1 g, C. 30.0 g PAUSE By now, you should know how to calculate the molar mass given a formula. CLICK For NH3, you should be able to calculate that the molar mass is 17.03 grams per mole. To do a mole-to-mass conversion, it is important to recognize that these two quantities are directly proportional. This means that the ratio of the two quantities is constant. In this case, The ratio of the mass of ammonia CLICK to the number of moles CLICK CLICK is equal to the ratio of 17.03 grams to one mole We can rearrange this equation to solve for the mass of ammonia. Essentially, what we would end up doing is multiply the given moles of ammonia, 2.00 moles CLICK by the ratio... 17.03 grams ammonia over 1 mole ammonia CLICK CLICK CLICK The ratio serves as a conversion factor. SHOW CANCELLATION OF UNITS The answer we get should be rounded off to three significant figures... since the the least precise factor, 2.00 CALLOUT “3 significant figures” has only three significant figures. 17.03 grams has four significant figures, CALLOUT and the number 1 in the denominator is exact. Two times 17.03 gives us 34.06, which we round off to 34.1. Therefore, the correct answer is choice B....34.1 grams. Here’s another way of looking at this calculation. CLICK if we want to convert moles to grams... we simply multiply the number of moles HIGHILIGHT by the molar mass HIGHLIGHT CLICK 2.00 moles, times 17.03 grams per mole, gives us 34.1 grams. CLICK PAUSE END RECORDING 17.03 g NH3 2.00 mol NH3 x = 34.1 g NH3 1 mol NH3 To convert moles to grams: Mass = moles x (Molar Mass) = 2.00 mol x (17.03 g/mol) = 34.1 g

Video ID: 4-5-3 © 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