How many formula units are in 2 moles of H2SO4? How many molecules are in 55g of H2O?

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We can solve both of these problems by looking at the units and using known relationships. If we want to know how many formula units there are in 2 moles of H2SO4, the first thing we need to realize is what a mole means. Just like a pair means 2 and a dozen means 12, a mole measns we have a specific number of something. While there is a scientific explanation for the value of a mole, ultimately it just means that we have 6.022 x 10^23 of whatever we are talking about.

If we have 2 moles of H2SO4, then we can set up the calculation as follows

2 moles H2SO4 (6.022 x 10^23 units / 1 mole) = 1.204 x 10^24 units of H2SO4

For the second question, we have to look at the units we are given, grams, and ask how do we get grams to moles?  This is where we use the molar mass for water (18.01 g/mol) to convert from grams to moles.  Then we can use the value of a mole to convert from moles of water to molecules of water

55 g (1 mol / 18.01 g)(6.022 x 10^23 molecules / 1 mol) = 1.8 x 10^24 molecules of H2O

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