Question

Calculate the number of molecules present in 42 grams sodium hydrogen carbonate (NaHCO₃).
(Na=23, H=1, C=12, O=16)

Answer 1

Step 1 - identify the atomic mass of carbon

Step 2 - calculate the atomic mass of the compound

Step 3 - work out the percentage, by mass, of carbon in the compound

Method:

Step 1:

To calculate the atomic mass, a periodic table is required. The atomic mass of each element is located at the top of the element, displayed as a number. In this case carbon has an atomic mass of 12 g/mol.

Step 2:

The same principles in step 1 are required and applied to the remaining atoms in the compound - sodium hydrogen carbonate. The atoms in the compound are sodium (Na), hydrogen (H), carbon (C) and oxygen (O). Looking at the value of atomic mass on the periodic table, the values of atomic mass are as follows:

Na = 23 g/mol

H = 1 g/mol

C = 12 g/mol

O = 16 g/mol

There is 1 Na atom, 1 H atom, 1 C atom and 3 O atoms in the compound. Now add these values together to get the atomic mass of the compound as such:

23+1+12+(3*16) = 84 g/mol

Step 3:

Now the percentage, by mass, needs to be calculated. The general rule for calculating a percentage is:

percentage = (value/total)*100%

Therefore by applying this rule for our problem we find that

Percentage = (12/84)*100% = 14.3 % (to 1 d.p) this way u need to do that