Adevice for measuring atmospheric pressure is a

Barometer

The device used in meteorology to measure atmospheric pressure is called a barometer. It consists of a long glass tube filled with mercury and turned upside down into a container of mercury. The tube is closed at one end and open at the other which works by balancing the mercury in the glass tube against the outside air pressure. As air pressure increases, the air becomes heavier and it pushes more of the mercury up into the tube. As air pressure decreases, mercury drains from the tube. 

"you start with 15.67 g mgco3-xh2o, when heated and all the water is removed, you are left with 7.58 g mgco3 only.

therefore, you had 15.67 - 7.58 = 8.09g h2o  

moles of each :

7.58g mgco3 = ? moles = 7.58g / 83.4g/mole = 0.09moles  

8 09g h2o = ? moles = 8.09g / 18g/mole = 0.44moles  

molar ratio between the two by dividing both numbers by the smallest number :

0.09 / 0.09 = 1  

0.44 / 0.09 = 4.8  

mgco3-5h2o"