**RAUL LAW** (F. M of Raoult, the fr. chemist and the physicist, 1830 — 1901) — the physical law, according to Krom the partial pressure of a saturated steam of solvent over solution is proportional to a molecular ratio of solvent and does not depend by nature solute. River z. finds broad application in medical - biol. researches at measurement of osmotic pressure in biol. liquids (see. Osmotic pressure ), etc. The law was open and formulated by Raul in 1886.

Mathematically R. z. express the equation

P = P_{ 0 } * X_{ 0 } ,

where P — pressure of a saturated steam of solvent over solution; R_{ 0 } — pressure of a saturated steam of pure solvent; X_{ 0 } — molecular ratio of solvent, equal

n_{ 0 } / (n_{ 0 } + n)

where n_{ 0 } and n — number of moths of solvent and solute respectively.

Size ΔP = (R_{ 0 } — P) call pressure decline of steam of solvent over solution.

RAUL the LAW is fair for the diluted solutions of nonelectrolytes; for the concentrated solutions observe more or less considerable deviations from R. z.

When solute is electrolyte, i.e. at dissolution dissociates on ions, and the quantity of kinetic active particles thus increases, pressure decline of steam of solvent over solution of electrolyte ΔP_{ E } in i times more pressure decline of steam of solvent over solution of a nonelectrolyte of the same concentration, i.e. ΔP_{ E } = iΔP.

Size i is called coefficient of isotonicity, or Vant Hoff's coefficient (see. Dissociation, in chemistry ).

Relative pressure decline of steam of solvent over solution (P_{ 0 } - P)/P_{ 0 } to equally molecular ratio of solute, i.e.

P_{ 0 } - P)/P_{ 0 } = n/(n_{ 0 } + n)

For the diluted solutions size n is small in comparison with size n_{ 0 } , and in a denominator it can be neglected. Then

P_{ 0 } - P)/P_{ 0 } = n/(n_{ 0 } ), but shopping mall of n = m/M and n_{ 0 } = m_{ 0 } / M_{ 0 } ,

where m and m_{ 0 } — weight quantities, and M and M_{ 0 } — pier. weight (weight) of solute and solvent respectively, thus

P_{ 0 } - P)/P_{ 0 } = (mM_{ 0 } ) / (m_{ 0 } M)

Using the last equation, it is possible to calculate a pier. the weight of solute, having measured P and P_{ 0 } . This way sometimes apply a pier to definition. weight of biologically active compounds.

**Bibliography:** Course of physical chemistry, under the editorship of Ya. M. Gerasimov, t. 1, page 175, etc., M., 1969; Poling of L. both P about l and N of of P. Himiya, the lane with English, page 267, M., 1978; R and about u 1 t F. M of Cryoscopie, R., 1901.

*N. B. Sametskaya.*