7. Gases
Standard Temperature and Pressure
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Here in this example question it says, a sample of oxygen gas has a measured volume of 325 ml's at STP. How many grams are present? Alright. So here they're giving us volume in the forms of milliliters, and s t p is giving us temperature plus pressure. We know from the ideal gas law with these values given to us or these variables given to us, the only thing that's missing is our moles because we have volume already, we have pressure and temperature, and we always know what r is. So we're gonna divide up r t and when we do that we're gonna get our moles. Moles here will equal pressure times volume over r times t. At s t p, our pressure is 1 atmosphere. Our volume, we just change milliliters to liters, so 0.325 liters. We have our r constant which is 0.08206 liters times atmospheres over moles times k. Then remember we're dealing with temperature at s t p, we use the units of Kelvin, so that's 273.15 Kelvin. So here, Kelvins cancel out, liters cancel out, atmospheres cancel out, and we'll be left with the moles of our oxygen gas. When we plug that in, we get 0.01450 moles of 0 2. But here the question is not asking us to determine the moles of oxygen gas, but instead the grams of oxygen gas. So we just need to do a simple conversion. We say that for every 1 mole of o 2 it weighs weighs 32 grams because there's 2 oxygens. Those cancel out and what we get at the end is 0.46 4 grams of 0 2. Here our answer has 3 significant figures because the value of 325 has 3 significant figures as well. So just the value that's missing is moles. Then go for moles that the value that's missing is moles. Then go from moles to grams to get your final answer.
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