A solution of HCl of volume 400 cm3 and concentration 0,25 mol.dm-3 is diluted until the concentration decreases to 0,1 mol.dm-3. What quantity remains the same before and after diluting?

Answer 1

The number of moles of the solute (HCl) remains the same before and after diluting.

Before diluting:
Volume of solution = 400 cm3 = 0.4 dm3
Concentration of HCl = 0.25 mol.dm-3

Number of moles of HCl before diluting = Volume x Concentration
Number of moles of HCl before diluting = 0.4 dm3 x 0.25 mol.dm-3
Number of moles of HCl before diluting = 0.1 mol

After diluting:
Concentration of HCl = 0.1 mol.dm-3

Number of moles of HCl after diluting = Volume x Concentration
Number of moles of HCl after diluting = 0.4 dm3 x 0.1 mol.dm-3
Number of moles of HCl after diluting = 0.04 mol

Therefore, the quantity that remains the same before and after diluting is the number of moles of HCl, which is 0.1 mol.