22.21 = mass paper + first lump NaOH
-0.63 = mass paper
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21.58 = mass NaOH
Now you tare the 22.21 (paper+NaOH) to zero.
The second lump adds 20.64 the third lump + second lump = 38.98
So total NaOH = 38.98 + mass first lump of 21.58 for total of 38.98+21.58 grams. You convert to the other units.
By the way, the person making the problem should have used some material other than NaOH since solid NaOH absorbs CO2 and H2O from the atmosphere and besides corroding the balance pans will continually add moisture and CO2 from the air and will not have a stable weight.
You place a piece of paper on a balance and find that its mass is 0.63 g. You add a lump of NaOH to the paper and get a mass of 22.21 g. You remember that you forgot to tare the balance before taking this measurement. You tare the balance, add another lump of NaOH, and read 20.64 g. You add another lump of NaOH and read 38.98 g. How Much NaOH do you have on the balance, in grams, milligrams, micrograms and kilograms?
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