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Let’s say if I have a known mass (10g) of an inorganic compound (NaCl) in its solid form and I dissolve it in water to form an NaCl solution and the total mass of the solution, let’s say 100g.

if I want to calculate the number of moles of the NaCl in the solution, does the number of moles change when i dissolve it from its solid to liquid form ?

Or is the number of moles is still = 10 / molecular weight

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Number of Moles will not change if Solid doesn't react with solvent. Here, NaCl is stable in water and solution will be 10% w/w. (10 g NaCl in 100 g solution).

If you want to calculate number of moles in 10% w/w solution, for 10 g of solution Number of moles = (10 g solution/ NaCl molecular weight) / 10

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No. Number of moles does not change until and unless NaCl reacts with water which it won't. No of moles of any substance= amt of substance( in gms)/ molecular weight of that substance( in gms). If you dissolve a substance in anything, its molecular weight does not change and the amt you are dissolving does not change. It does not matter if its physical state changes or not. The no of moles remains constant. You may keep adding solvent but as long as you do not add some more NaCl into the solution your no of moles of NaCl will never change. But be careful while adding solvent since it will change your concentrations. The reason why i mentioned that as long as there is no chemical reaction the no of moles will remain constant is because if there is a chemical reaction, there will be formation of products at the expense of ractants and we know that mass of reactants decreases and mass of products increases. Thus amt of NaCl will decrease in case of a reaction and change your no of moles in case of a chemical reaction involving NaCl.

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