One of relatively few reactions that takes place directly between two solids at room temperature is

Ba ( OH ) 2 +   8 H 2 O ( s )   +  NH 4 SCN ( s ) → Ba ( SCN ) 2 ( s )   +  H 2 O ( l )   +  NH 3 ( g )

In this equation, the · 8H₂O in Ba(OH)₂ · 8H₂O indicates the presence of eight water molecules. This compound is called barium hydroxide octahydrate.

a. Balance the equation.

b. What mass of ammonium thiocyanate (NH₄SCN) must be used if it is to react completely with 6.5 g barium hydroxide octahydrate?

Interpretation: The balanced form of the given chemical equation is to be stated. The amount of ammonium hydroxide that must be used, for it to react completely with the given amount of barium hydroxide is to be calculated.

Concept introduction: The mass of a substance can be obtained by using the number of moles of the substance present and its molar mass. The formula used to calculate the mass of a given substance is,

Mass of the substance=(Number of moles)×(Molar mass of the substance)

To determine: A balanced form of the given chemical equation.

According to the law of conservation of mass, mass can neither be created nor destroyed. The mass of reactants is equal to the mass of products formed. Therefore a chemical equation, having lesser number of moles of an element on either side of a reaction, is balanced using appropriate numerical coefficients to satisfy the law of conservation of mass.

Adding the coefficient 2 to NH3 and the coefficient 2 to NH4SCN balances the number of atoms of nitrogen present on either side of the reaction. The equation thus obtained is,

Ba(OH)2⋅8H2O(s)+2NH4SCN(s)→Ba(SCN)2(s)+H2

Interpretation: The balanced form of the given chemical equation is to be stated. The amount of ammonium hydroxide that must be used, for it to react completely with the given amount of barium hydroxide is to be calculated.

Concept introduction: The mass of a substance can be obtained by using the number of moles of the substance present and its molar mass. The formula used to calculate the mass of a given substance is,

Mass of the substance=(Number of moles)×(Molar mass of the substance)

To determine: The amount of ammonium hydroxide required to react completely with the given amount of barium hydroxide.