amount of substance

• what are the 3 equations for moles?


  moles = mass/mr. moles = conc x vol. pV=nRT
• what is a standard solution?

  the concentration is definitely and accurately known, and which is made up by dissolving an accurately known mass of the substance in a known volume of pure water.
• what is practical 1?

  titrations.
• what is step 1 for making up a standard solution?

  place a clean and dry weigh boat and record the mass.
• what is step 2 for making up a standard solution?

  add the calculated mass of solid needed to make the standard solution to the weight boat and record the mass.
• what is step 3 for making up a standard solution?

  transfer the contents of the weigh boat into a clean and dry beaker reweigh the weigh boat as some of the solid may remain inside.
• what is step 4 for making up a standard solution?

  subtract the mass recorded in step 3 from the mass recorded in step 2 to calculate the accurate mass of solid transferred to the beaker.
• what is step 5 for making up a standard solution?

  add a small amount of deionised (or distilled) water to the beaker and use a stirring rod to stir the contents until all of the solid has dissolved into the water.
• what is step 6 for making up a standard solution?

  use a small funnel to transfer the contents of the beaker into a volumetric flask. Rinse the beaker and stirring rod using deionised water and transfer the washings into the volumetric flask. You may have to do this a couple times to make sure that all of the solution has been transferred and none remains in the beaker or on the stirring rod.
• what is step 7 for making up a standard solution?

  add deionised water to the volumetric flask until the meniscus of the solution is just below the graduation line on the neck of the flask.
• what is step 8 for making up a standard solution?

  add the deionised water dropwise using a pipette until the bottom of the meniscus is exactly on the graduation line. If you add too much and the meniscus is higher than the graduation line, the volume of solution is no longer known, and the concentration is no longer accurate.
• what is step 9 for making up a standard solution?

  add the stopper to the top of the volumetric flask and invert the flask several times to ensure that the solution is thoroughly mixed.
• what might you have to do for dilutions of solutions?

  to decrease the conc of a standard solution by adding a specified volume of water to dilute it
• what may you be asked in an exam with dilutions of solutions?

  to calculate the new conc of a solution after a specified volume of water has been added or calculate the volume if water needed to dilute the solution down to a specific conc.
• in a dilution, we decrease the conc of the solution only by adding water, does this change the number of moles?

  no, the number of moles before and after dilation is the same.
• what is the equation for conc and volume before and after a dilution?

  (C_AV_A = C_BV_B). C_A = conc before. V_A = vol before. C_B = conc after. V_B = vol after.