Lehman College of the City University of New York Department of Chemistry 15)Par

Question

Lehman College of the City University of New York Department of Chemistry 15)Paraffin wax has the chemical formula CauHes and has no dipole moment. P roo m tem perature but water is a liquid. These phase differences are best explained by: a The IMFs b. The IMFs in paraffin are weaker than the IMFs in water making paraffin a solid at room c The IMFs in paraffin are weaker than the IMFs in water. Paraffin should therefore be a d. The IMES in pnrwater and formsmany strong hy s in paraffin are stronger than the IMFs in water. The only IMFs in paraffin are weak London dispersion forces but paraffin is such a large molecule that the sum of the weak LDFs is greater than the hydrogen bonding in water. temperature and making water a liquid because water is so much smaller. liquid but it is an anomaly in chemistry. hydrogens than water and forms many strong hydrogen bonds s in paraffin are stronger than the IMFs in water. Paraffin has many more 16)The molecular model to the right represents the bonding in a AB C2 molecule corresponding to the accompanying Lewis structure. The B atoms are in the axial position and the C atoms are in the equatorial position. For this molecule the BA-C bond angle is and the CA-C bond angle is a. exactly 90°, slightly less than 120 b. slightly less than 90, exactly 120 c. exactly 90, exactly 120° slightly less than 90°, slightly less than 120 H H :0: For questions 17 and 18, consider the molecule cysteine (HSCH2CHNH2COOH), a semi-essential amino acid with a molar mass of 121 g/mol and the Lewis structure shown to the right: H-S-c- H N-H 17)lf you have 150. mg of cysteine, how many moles of cysteine are present in your sample? a. 7.47x1020 moles b. 807 moles C. d. 0.807 moles e 1.24x103 moles CHsCH2CHCHsCOOH shown to the right. Which one of HS-H these molecules would you expect to have a higher vapor pressure at room temperature and why? a. Cysteine because it is a polar molecule b. CHsCH2CHCHsCOOH because it has much stronger LDF interactions c. Cysteine because it can form more hydrogen bonds d CHsCH2CHCHsCOOH because it can form fewer hydrogen bonds e· Cysteine because it has much weaker LDF interactions 18) Now compare cysteine to the following organic molecule

Explanation / Answer

15) Answer A

16) Answer C (exactly 90 and 120 0)

17) Answer E (0.150/121 = 1.24*10^-3 mol)

18) Answer D ( less H-bonds)

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