Hi.
You can calculate mass percent by dividing the mass by the total mass of the compound & multiplying by 100.
28.8 ÷ (171.2 + 28.8) = 0.144
0.144 · 100 = 14.4
I believe the mass percent of the hydrogen in the compound is 14.4%
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"any of a large class of chemical compounds in which one or more atoms of carbon are covalently linked to atoms of other elements, most commonly hydrogen, oxygen, or nitrogen. The few carbon-containing compounds not classified as organic include carbides, carbonates, and cyanides. See chemical compound."
An educated guess is called estimation
It is homogeneous mixture of two non reactive substance.
AT, STP in chemistry is the abbreviation for Standard Temperature and Pressure. STP most commonly is used when performing calculations on gases, such as gas density