Creating a Positive Learning Environment for Everyone Be respectful of your classmates Save conversation for clicker portions of lecture Put cell phones in “airplane” mode Turn off tablets Close laptops

Announcements Lon-capa HW #2 (Type 2) due tomorrow by 7:00 p.m. Prelecture assignment and textbook homework due as usual Lab write-up for lab this week is not due until next Wednesday, September 20th. Read Chapter 13 (gases) before lab tomorrow.

Clicker #1 You have two beakers on your lab table. Beaker #1 contains 32.07 g of sulfur and Beaker #2 contains 74.92 g of arsenic (As). Which beaker contains the greatest number of atoms? Choose the best answer. A) Beaker #1 because one sulfur atom weighs less than one arsenic atom so you need more of the sulfur atoms to fill the beaker. B) Beaker #1 because it contains more moles of sulfur atoms versus the number of moles of arsenic atoms in Beaker #2. C) Beaker #2 because arsenic has a greater mass than sulfur. D) Beaker #2 because it contains more moles of arsenic atoms versus the number of moles of sulfur atoms in Beaker #1. E) Beakers #1 and #2 contain the same number of atoms because there is one mole in each.

Example How many H2O molecules are contained in 50.0 g of water? How many hydrogen atoms are present in this sample?

Clicker #2 Does a methane molecule (CH4) consist of more hydrogen or carbon? A) hydrogen B) carbon C) depends

Clicker #3 What is the percent composition (by mass) of carbon in 2 moles of CH4? A) 2.00% C B) 37.45% C C) 74.9% C D) 149.8% C

Another Example What is the percent mass of nitrogen in barium nitrate?

One more… What is the percent mass of oxygen in each? H2O vs. H2O2

New Example You have some “nitrogen oxide” compound and you want to figure out what it is (both the formula and the name). You know it’s 30.4% nitrogen by mass and the molar mass is 92.0 g/mol. NxOy = formula? name?

Clicker #4 The molecular formula (actual formula) for water is H2O. What is its empirical formula? A) HO½ B) H2O C) H2O2 D) H4O2 E) Cannot be determined without knowing the percent mass of each.

Consider the problem… If you have equal masses of each compound (H2SO4, C12H22O11, KClO3), which one has the greatest number of oxygen atoms?