According to Mendeleev's periodic table, what was the atomic mass of Eka-aluminium which was later replaced by Gallium?

68

The correct answer is option 3. 68.

Eka-aluminium was a placeholder name given by Dmitri Mendeleev in his periodic table to an undiscovered element that he predicted would have properties similar to aluminium. He left gaps in his periodic table for elements that were yet to be discovered but would fit into the overall pattern of the table.

Mendeleev predicted the properties of eka-aluminium based on the properties of neighbouring elements in the periodic table. One of the key predictions was the atomic mass of eka-aluminium. In the case of eka-aluminium, Mendeleev predicted its atomic mass to be approximately 68. Later, in 1875, French chemist Paul-Émile Lecoq de Boisbaudran discovered an element with properties close to Mendeleev's predictions. This element was named Gallium, and its atomic mass was found to be close to the predicted value of 68.

So, the prediction of eka-aluminium with an atomic mass of 68 was later confirmed by the discovery of Gallium. The successful predictions of unknown elements in Mendeleev's periodic table provided strong support for the validity of the periodic law and the arrangement of elements based on their properties.