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density = mass/volume

therefore, mass = density x volume

so if you have 20ml of acetone you have 20 x 0.787 g

divide this by the molar mass of acetone to get moles
= 15.68/58 = 0.270 moles

this number of moles in 200ml is the same concentration as 0.270/0.2 moles/litre

= 1.352 Molar
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