# Calculate the mass fraction (%) of the following substances: a) Na2CO3; b) Ca (H2PO4) 2?

Calculate the mass fraction (%) of the following substances: a) Na2CO3; b) Ca (H2PO4) 2? How many grams of copper is in 5 mol: a) CuO; b) Cu (OH) 2?

The mass fraction of individual atoms of elements in the composition of a substance is as follows:

w% = Mr (element): Mr (total substance) * 100%.

Using this formula, we find the mass fraction of elements in the presented compounds:

a) Na2CO3:

w% (Na) = (2 * 23): Mr (Na2CO3) * 100% = 46: 106 * 100% = 43.4%

w% (C) = 12: 106 * 100% = 11.3%

Then oxygen accounts for 100% – (43.4% + 11.3%) = 100% – 54.7% = 45.3%

b) Ca (H2PO4) 2:

w% (Ca) = 40: 234 * 100% = 17.1%

w% (H) = (4 * 1): 234 * 100% = 1.7%

w% (P) = (2 * 31): 234 * 100% = 30.4%

w% (O) = (8 * 16): 234 * 100% = 54.7%

Now we find the mass of substances, based on the molar amount:

a) m (CuO) = n * M = 5 mol * 80 g / mol = 400 g.

b) m (Cu (OH) 2) = n * M = 5 mol * 98 g / mol = 490 g.

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