What is the mass of #9.75xx10^26# #"lithium atoms"#?

Well first, we need to convert #9.75*10^26# atoms of lithium into moles of lithium. Note that, #1mol=6.02*10^23# atoms.

#:.# There are #(9.75*10^26)/(6.02*10^23)=1619.6~~1620# moles of lithium.

Now to find the mass, we can use the molar mass of lithium and multiply it by the number of moles.

#"mass"="molar mass"*"moles"#

Lithium has a molar mass of #6.941g"/"mol#.

#:.# Its mass would be #1620cancel(mol)*6.941g"/"cancel(mol)=11244.42g#

We now need to round to three significant figures.

#:.=1.12*10^4g#

You need to convert atoms Li to moles Li by multiplying by #("1 mol Li )/(6.022"xx"10"^23" atoms Li")#. This gives you moles Li. Then multiply moles Li by the molar mass of Li #("6.94 g/mol" -"from the periodic table")#.

Moles Li

#9.75xx10^26color(red)cancel(color(black)("atoms Li"))xx(1"mol Li")/(6.022xx10^23color(red)cancel(color(black)("atoms Li")))=1.619xx"10"^3" mol Li"#

Mass Li

#1.619xx10^3color(red)cancel(color(black)("mol Li"))xx(6.94"g Li")/(1color(red)cancel(color(black)("mol Li")))=1.12xx"10"^4" g Li"# rounded to three significant figures

You can combine the two steps into one equation.

#9.75xx10^26color(red)cancel(color(black)("atoms Li"))xx(1color(red)cancel(color(black)("mol Li")))/(6.022xx10^23color(red)cancel(color(black)("atoms Li")))xx(6.94"g Li")/(1color(red)cancel(color(black)("mol Li")))=1.12xx"10"^4" g Li"#