So 28 grams of graphene could cover 28 football fields. Why not say that 1 gram of it could cover 1 football field? Or 142,857 grams of it could cover 142,857 football fields?
Well, basically one ounce is about 28 grams, so this article converted the ounce into grams for its international audience. The person who did the conversion didn't notice the cancellation (or did but was forbidden from making it for some reason). Journalists aren't renowned for their arithmetic skills.
Hmm. Carbon has an atomic weight of 12.0107, so treating the carbon atoms that make up graphene individually (which may or may not make sense), a mole of carbon weighs 12.0107g. There are 6.022 x 1023 atoms in a mole. A gram of carbon therefore has (6.022 x 1023)/12.0107 = 5.01386264 x 1022 atoms in it.
The separation of atoms in graphene is the same as in a layer of graphite: 0.142nm, or 1.42 x 10-10m. They're laid out in a lattice hexagonally. The number of hexagons is half the number of atoms (because a hexagon has 6 sides and each corner (atom) is part of 3 hexagons), so 5.01386264 x1022 atoms means 2.50693132 x 1022 hexagons. The area of a hexagon with a side length L is (3 x 3½ x L2)/2. Therefore the area of 2.50693132 x1022 hexagons with a side length of 1.42 x 10-10m is (2.50693132 x 1022 x 3 x 3½ x (1.42 x 10-10)2)/2 square metres.
So that's about 1313.32137 square metres give or take some rounding errors. This is about the same as a square with sides just over 36m long.
Hmm, seems a bit small for a football field. I wonder where I made my mistake?