Describe the procedure and the equipment needed to make.. ?

500.0 ml of 0.102 M NaOH using sodium hydroxide pellets

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  • 10 years ago
    Favorite Answer

    That's pretty standard. (NaOH always come in pellets).

    The exercice's objective is to test your capacity to translate moles into weight and Viceversa.

    The Molecular weight of NaOH is 40.00g/mol

    1L (1000 mL) of 0.102 M (M means mol per liter or mol/l) NaOH contains well 0.102 mole of NaOH.

    Since NaOH weights 40.00g/mol there is 40.00g/molx0.102mol = 4.08g of NaOH per 1000 mL of the solution.

    (did you notice how the units cancel each other: grams divided per moles multiplied per moles gives grams..... This is how you can check you are doing things right: verify your units ALWAYS).

    So to prepare 500 mL (1/2 a liter) you need 4.08/2= 2.04 g of NaOH.

    Practical note: Pellets is the common way NaOH comes in (simply because it is hygroscopic that is it sucks humidity out of the air or anything).

    In powder form it would just become a big unmanagable lump.

    So when you prepare a solution you actually seldom arrive at the exact weight (i.e.: 4.10 instead of 4.08).

    Truth as the other contributor said NaOH isn't pure.

    So once you are done preparing your solution it is standard procedure to do what is called a titration.

    I won't bother going in the details since this look like a typical "solution/analytical chemistry" question which means you already know what titration is about.

    Hope this helps

    Bluebird

  • Colin
    Lv 7
    10 years ago

    to make 1L of 0.102M NaOH needs 40 x 0.102g

    to make 500mL of 0.102M NaOH needs 40/2 x 0.102g

    You need to use analytical grade AR NaOH for this, not ordinary lab stuff which isn't pure enough. Ideally start with a brand new unopened bottle otherwise you will find the pellets may have already stuck together.

    1] take a stoppered weighing bottle and record its mass

    2] quickly add the required mass of NaOH

    This needs to be done quickly as NaOH is hygroscopic and also reacts with CO2 in the air.

    3] put about 200mL of distilled water in a clean beaker, and pour in the solid NaOH

    4] some NaOH will stick in the weighing bottle. Using a wash bottle, wash this into the beaker.

    5] now take your 500mL volumetric flask, and using a funnel, transfer the solution to the flask. Use your wash bottle to wash down the beaker then the funnel to make sure it is all transferred.

    6] now use the wash bottle to top up your volumetric flask to the 500mL mark. Mark sure it is topped up so the bottom of the meniscus just sits on the mark.

    7] stopper the volumetric flash and shake and invert it several times to ensure even mixing.

    8] Finally label the flask.

    Ideally, because NaOH absorbs moisture from the air very quickly, you should weigh out slightly more than calculated. Also, because it is pellets, you may not be able to get the exact mass to plus or minus one pellet. Then proceed as before and make up your [too strong] solution. You would then have to standardise a sample against 0.100M HCl, titrating using methyl orange indicator [or similar]. Calculate the exact concentration from the titration. Then measure out a sample and calculate how much water you need to add to make it exatly 0.102M using M1V1 = M2V2.

    However, as you are not sure even how to make the basic solution, I suspect your teacher won't expect you to know about the second part?

    Source(s): retired chemistry examiner
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