Percent mass of elements in compounds ( Nov. 12 '09)

Find the % of carbon by mass in ethane (C2H6)
2(12g/mol)+6(1.0g/mol) = 30 g/mol
% Carbon = 24g/30g = 80%
% Hydrogen = 6g/30g = 20%

Percent composition
- means the % mass of each element in a compound
Find the composition of K2Cr2O7
K2 x 39.1 = 78.2 (27%)
Cr2 x 52 = 104 (35%)
07 x 16 = 112 (38%)
======294.2

(leave 1 decimal place for accuracy)

Finding the mass of an element in a given mass of a compound.
Find the mass of Carbon contained in a 25.0 g sample of CO2
C1 x 12 = 12 (27.3%) --> .2727(25g) = 6.8 g
O2 x 16 = 32 (72.7%) --> .7272(25g) = 18.2 g
====== 44 ================== 25g

Find the mass of K, C, and O contained in K2CO3 if the sample is 450.0 g.
K2 x 39.1 = 78.2 ( 56.6%)(450g) = 254.7 g
C1 x 12 = 12 (18.7%)(450g) = 39.6 g
O3 x 16 = 48 (34.7%)(450g) = 156.2 g
====== 138.2===========450 g

other examples:
Percent composition of C3H8
C3 x 12 = 36 (81.8%)
H8 x 1 = 8 (18.2%)
===== 44=100%
Find the percent of 2H2O in CaCl-2H2O
Ca1 x 40.1 = 40.1
Cl2 x 35.5 = 71
44 x 1 = 4 (2.7 %)
O2 x 16 = 32 (21.8 %)
====== 147.1 == 24.5%