Sea water is modelled as $0.60 M$ $NaCl$ (complete dissociation) at $298 K$ . Using $R = 0.0821 L atm mo l^{- 1} K^{- 1}$ , the minimum external pressure that must just be exceeded for reverse osmosis is about

Answer: D

The applied pressure must exceed

π = i CRT

. With

i = 2

π = 2 \times 0.60 \times 0.0821 \times 298 = 29.4

atm. Taking

i = 1

would give only half this value.