A sample of river water contains both suspended clay particles and dissolved sodium…
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A 1.000L sample of river water contains both suspended clay particles and dissolved sodium chloride. The initial composition is given in the table below.
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Species Mass present (g) Average particle diameter
Clay 0.50 1μm
NaCl 2.00 hydrated ions ≈0.7nm
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The water is first passed through a depth sand filter that effectively removes particles larger than 5μm, then through a membrane filter with nominal pore size 0.45μm, and finally through a nanofiltration membrane with pores of 50nm average diameter. Assume the nanofiltration membrane retains 95% of dissolved NaCl by mass.
(a) For each of the three filtration stages, identify which of the two species (clay particles, dissolved NaCl) is removed to a significant extent. Support your answer by comparing particle or ion diameters to pore sizes.
(b) Calculate the mass of clay and the mass of NaCl remaining in the water after all three stages are complete.
(c) Explain why, despite the much smaller pore size of the nanofiltration membrane compared with the 0.45μm filter, it is still incapable of achieving complete (100%) removal of dissolved NaCl.
(d) Propose one modification to the separation sequence that could increase the NaCl removal to at least 99.5%. Justify your choice in terms of the property exploited by the modification.