an element E forms a hydride EH3, which contains 90% of E by mass. what is the relative atomic mass ?​

27 g/mol of E

Explanation:

Note that percentage by mass= mass of each element present. So, since there is 90% of E, there is 90g of E present. By implication, there are 10g of H corresponding to 10%H. Note that there is 100g of EH3

1 moles of E corresponds to 90 g of E

Mole ratio of E: H= 1:3

Thus

Number of moles of H = 10g/ 1g/ mol = 10 moles of H

Since E contains 1/3 the number of moles of H

Number of moles of E = 1/3 × 10 = 3.33 moles of E

Molar mass of E= mass of E/ number of moles of E

Since mass of E = 90 g

Molar mass of E = 90g/3.33 moles

Molar mass of E = 27 g/mol