Many chemical salts come as hydrated crystals. That is, there is a specific ratio of water molecules associated with the salt crystals
e.g. MgSO4.7H2O (called Magnesium sulphate heptahydrate FW = 246.5) has 7 water molecules to every Mg2+ and SO42- ion pair.
But it is also possible to get MgSO4 as an anhydrous powder (without water) FW = 120.4. Once dissolved in water these salts have the same chemical properties but in the solid state they obviously have a different FW.
You are given a recipe for a buffer that contains 5% (w/v) MgSO4.7H2O. However, you only have MgSO4 anhydrous available to you. How much MgSO4 anhydrous do you need to make 0.5L of the buffer with the same molar concentration as in the recipe?
2 answers
2.5
weel if u crry the 2.5 then do a backflip then it does it all