Magnesium sulphate water 7 on a watch glass. Magnesium sulfate heptahydrate; MgSO4?H2O; Molar mass 246.47 g/mol. Also known as Epsom salt, English salt, bitter salts, bath salts, epsomite. It is an??norganic??alt, commonly the heptahydrate sulphate??ineral??psomite, Epsom salt, which can relieve constipation but may also be applied to sprains, bruises and boils. As a monohydrate it is mainly used as a fertilizer in agriculture for correcting magnesium and sulphur deficiencies in the soil.