Theorie:
Study the main partitives.
Phrases with countable and uncountable nouns (pieces).
Remember!
A bar of chocolate
A bar of soap
A hunk of cheese
A chunk of rock
A cube of sugar
A block of ice
A slab of concrete 
A slab of concrete
A rasher of bacon
A clod of earth
A wad of banknotes
A slice of cake
A slice of bread
A dollop of jam 
A sheet of paper
A pane of glass
A blob of paint
A trickle of rain
A drop of sauce
A puff of perfume 
A wad of banknotes
A slice of cake
A slice of bread
A dollop of jam
A lump of butter
A sheet of paper
A pane of glass
A blob of paint
A trickle of rain
A drop of sauce
A puff of perfume
A wisp of smoke
A segment of orange
A pinch of salt
A chip of china
A shred of cloth
A square of chocolate 
A pinch of salt
A chip of china
A shred of cloth
A square of chocolate
Examples:
I want to eat a square of chocolate.
Look! There is a segment of orange on the plate.
Give me a sheet of paper, please.