The most notable of these are known as the "six simple machines": the wheel and axle, the lever, the inclined plane, the pulley, the screw, and the wedge, although the latter three are actually just extensions or combinations of the first three.

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how much sodium chloride must be added to 100 mL of water so that its concentration is 20 parts per million assume that the dens

OlgaM077 [116]

Answer: 2mg

Part per million is unit that equal to mg/kg of water or simply $10^{-6}$ . In this case, you asked how much NaCl needed and know the volume of water(100mL), concentration(20ppm or 20 mg/kg) and water density.
Then, the equation would be

concentration = NaCl weight/ water weight
20 x $10^{-6}$ mg= NaCl weight/ (100ml x 1g/ml)
NaCl weight= 20x $10^{-6}$ mg/kg x 100g
NaCl weight= 2000 x $10^{-6}$ g
NaCl weight= 2x $10^{-3}$ g = 2mg