Explain Why Mg Has A Higher Melting Point Than Na.

Type 1: metal vs metal
Example: Explain why Mg has a higher melting point than Na.
Reason: Compare the number of delocalized electrons.

Type 2: ionic compound vs ionic compound
Example: Explain why NaF has a higher melting point than NaCl.
Reason: Compare lattice energy.

Type 3: simple molecule vs simple molecule
Example: Explain why H2O has a higher melting point than H2S.
Reason: Compare the relative strength of the intermolecular forces.

Type 4: ionic compound vs simple molecule
Example: Explain why NaCl has higher melting point than H2O.
Reason: Compare ionic bond strength with weak intermolecular forces.

Type 5: giant molecule vs simple molecule
Example: Explain why SiO2 has a higher melting point than H2O.
Reason:

As it is the same for all gases, R is also called the universal gas constant.
R = 8.31 J K-1 mol-1
Assuming a gas sample to be under ideal conditions, the value of R can be determined by measuring its volume, temperature and pressure and substituting the values into the ideal gas equation.
Consider one mole of a gas at standard temperature and pressure (s.t.p).
E.g., P = 101 325 Pa, V = 22.4 x103 m3, n = 1 mol and T = 273.15 K
Using PV = nRT,

R = PV/nT = ((101325)(0.0224))/((1)(273.15)) = 8.31 J K-1 mol-1

Experimental Determination of the Molar Gas Constant

When magnesium is added to an aqueous solution of hydrochloric acid, the following reaction occurs:
Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
Preliminary Calculation (to determine the amount of reactants to be used)
With a graduated gas syringe of 100 cm3 capacity, assume that 80.0 cm3 of H2 gas is collected.

no. of moles of H2 gas = 80/24000 mol = no. of moles of Mg

mass of Mg = 80/24000 x 24.1 = 0.0803 g

Let the concentration of HCl(aq) used be 0.500 mol dm-3.

no. of moles of HCl = 80/24000 x 2