Atomic weight of gold is 197.2 ami calculate no of gram atoms in 7.5g of gold

The atomic weight is numerically equivalent to the molar mass.
therefore molar mass of gold is - 197.2 g/mol
avagadros number states that 1 mol of any substance is made of 6.022 x 10²³ units. These units could be atoms making up elements or molecules making up compounds. 
1 mol of gold is made of 6.022 x 10²³  atoms of gold
since 1 mol weighs 197.2 g 
therefore if 197.2 g of gold contains - 6.022 x 10²³  atoms of gold
then 7.5 g contains - 6.022 x 10²³  x 7.5/ 197.2 = 2.29 x 10²² atoms of gold 
answer is 2.29 x 10²² atoms of gold

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.

Density and pressure