public static ChemistryUnit
	parseExpression(String in){
		ChemistryUnit unit = new ChemistryUnit();
		in = in.replace(" ", "");
		String[] sa = in.split("\\+");
		for(String s : sa){
			String tempNumber = "";
			int counter = 0;
			char cati = s.charAt(counter);
			while(cati >= '0'&& cati <= '9'){
				tempNumber += cati;
				cati = s.charAt(++counter);
			}
			if(tempNumber.equals("")){
				tempNumber = "1";
			}
			int n = Integer.parseInt(tempNumber);
			if(n > 0){
				unit.putUnit(parseCompound(s.substring(counter)), n);
			}
		}
		return unit;
	}