About mole and molar calculations

Remember this fundamental formula:

Number of moles = Weight of substance / Gram molecular weight

As an example:
18g of water  =  1 mole of water because weight of water = 1 * 2 (H2) + 16 * 1 (O) = 18g
Therefore, Number of moles of water = 18g / 18g = 1 mole of water
Now, this 1 mole (18g) of water has 1 * 6.022 * 10^23 molecules of water in it

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ONE MOLE OF CARBON WEIGHS EXACTLY 12g

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Another example:
Calculate the number of moles in 4.5g of C6H12O6 (Glucose)

Gram molecular weight of glucose = 12 * 6 (C6) + 1 * 12 (H12) + 16 * 6 (O6) = 180
Number of moles = 4.5/180 = 0.025 moles