In an effusion experiment it required 40 s for a certain number of moles of a gas of unknown molar mass to pass through a small orifice into a vacuum. under the same conditions 16 s were required for the same number of moles of o2 to effuse. what is the molar mass of the unknown gas?

The molar  mass  of the unknown   gas  is  200  g/mol

calculation

let  the  unknown  gas  be represented   by  Y and molar mass be represented by  MM

The  molar mass of Y  is  therefore calculated  using  

 = (time of Y/ time of O2) =√(MM  of Y/ MM of O2 formula.

where  MM of  y =?

        MM of   O2 = 32  g/mol

       time  of Y = 40 s

      time   of O2 = 16  s

40/16 =√ (MM  of Y/ 32)

2.5=√MM   of Y/32

square  both side to remove the √ sign

6.25 =MM  of Y/32

multiply both side  by 32

MM of Y  is therefore= 200 g /mol

assuming that your periodic table has eighteen columns, you should highlight   the four elements in the third column from the left of the table.

neutral atoms of elements after calcium contain d electrons. the energy of d orbitals of a particular main shell lies between the energy of the s and p orbitals of the next main shell. for example, electrons in the have energy higher than those in the orbital while lower than those of the orbital.

by the aufbau principle, electrons fill orbitals one at a time, starting from those of the lowest energy. the orbital is therefore filled right after orbitals and before orbitals.  

potassium has electron configuration and calcium . the s orbital of a main shell holds a maximum of two electrons. the next element would thus have the electron configuration and satisfy the requirements.

a neutral atom of the element contains three electrons on top of the most recent noble gas element. it should thus be placed in the third column of the periodic table. both scandium and yttrium would satisfy the conditions. an aufbau diagram indicates that the 4f orbital is filled before the 5d. however, it appears that exceptions exists for both lanthanum and actinium. neutral atoms of the two d-block elements also demonstrate the and should be highlighted as well.