 Conversion between mass and moles of an element. Atomic weights can be used as conversion factors to determine the number of moles given the mass of a sample, or the mass of a sample given the number of moles. How many moles are there in 25.64 grams of iron? In order to solve this, we first need to find the atomic weight of iron from the table, atomic weights by name. The atomic weight of iron is 55.85 grams per mole. The atomic weight can be read as a conversion factor of either 1 mole of iron per 55.85 grams, or 55.85 grams per 1 mole of iron. To convert grams of iron to moles of iron, we need to use the appropriate conversion factor. In this case, the moles per gram conversion factor. Multiplying 25.64 grams by our conversion factor tells us that there is 0.4591 mole of iron in 25.64 grams of iron. What is the weight in grams of 0.326 mole of iron? We know that the atomic weight of iron is 55.85 grams per mole. So the conversion factor to use is either 1 mole of iron per 55.85 grams, or 55.85 grams per 1 mole of iron. To convert grams of iron to moles of iron, we need to use the appropriate conversion factor. Pause the video here and see if you can figure out which one to use. Think you got it? The correct conversion factor is the one on the right, because we need to cancel out the moles of iron in the denominator so that we are left with only grams of iron in the numerator. Doing the math, we know that the weight in grams of 0.326 mole of iron is 18.2 grams. Here's another example. What is the atomic weight of argon? Pause the video here. Play it once you think you know the answer. The correct answer is an atomic weight of 39.95 grams per mole. Try this one next. How many moles are there in 6.45 grams of tin to three significant figures? Pause the video here. Play it once you think you know the answer. There are 0.0543 mole of tin in 6.45 grams of tin. Let's do one more. How many grams are there in 0.100 moles of fluorine to three significant figures? Pause the video here. Play it once you think you know the answer. There are 1.90 grams of fluorine in 0.100 moles of fluorine. You have completed this activity. Conversion between mass and moles of an element.