When calculating concentration remember that in n = c * v, volume should be in dm3 so divide the volume by 1000 if it's in ml or cm3
Do you understand how to do the rest of the question, was it just the NaHCO3 conc you were struggling with?
By book goes through the answer and says that before you can treat it like a straightforward calculation you have convert the grams to moles and the 250cm3 to 1dm3. They do it like this:
250cm3 is quarter of 1000dm3 so to make up the solution with same concentration you would need four times as much as in 1 dm3, so you do 2.10 x 4 g dm3
= 8.40 g dm3
You now have the volume units right so convert grams to moles, so the concentration of NaHCO3 is 0.100 mol dm3, as 1 mole weighs 84g.
By book goes through the answer and says that before you can treat it like a straightforward calculation you have convert the grams to moles and the 250cm3 to 1dm3. They do it like this:
250cm3 is quarter of 1000dm3 so to make up the solution with same concentration you would need four times as much as in 1 dm3, so you do 2.10 x 4 g dm3
= 8.40 g dm3
You now have the volume units right so convert grams to moles, so the concentration of NaHCO3 is 0.100 mol dm3, as 1 mole weighs 84g.
You calculate the number of moles in the mass using n=m/mr, then apply that to n=c*v. When the volume is 1dm3, v just equals one, so the number of moles is equal to the concentration in moldm-3